Add dedicated cache method for creating build directories (#8910)

## Summary

Based on feedback from
https://github.com/astral-sh/uv/pull/8905/files#r1833531812.

crates/uv-build-frontend/src/lib.rs

@@ -261,7 +261,7 @@ impl SourceBuild {
         level: BuildOutput,
         concurrent_builds: usize,
     ) -> Result<Self, Error> {
-        let temp_dir = build_context.cache().environment()?;
+        let temp_dir = build_context.cache().venv_dir()?;
 
         let source_tree = if let Some(subdir) = subdirectory {
             source.join(subdir)

crates/uv-cache/src/lib.rs

@@ -188,8 +188,14 @@ impl Cache {
         self.bucket(CacheBucket::Archive).join(id)
     }
 
-    /// Create an ephemeral Python environment in the cache.
-    pub fn environment(&self) -> io::Result<tempfile::TempDir> {
+    /// Create a temporary directory to be used as a Python virtual environment.
+    pub fn venv_dir(&self) -> io::Result<tempfile::TempDir> {
+        fs_err::create_dir_all(self.bucket(CacheBucket::Builds))?;
+        tempfile::tempdir_in(self.bucket(CacheBucket::Builds))
+    }
+
+    /// Create a temporary directory to be used for executing PEP 517 source distribution builds.
+    pub fn build_dir(&self) -> io::Result<tempfile::TempDir> {
         fs_err::create_dir_all(self.bucket(CacheBucket::Builds))?;
         tempfile::tempdir_in(self.bucket(CacheBucket::Builds))
     }

crates/uv-distribution/src/source/mod.rs

@@ -1720,10 +1720,10 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
         }
 
         // Build into a temporary directory, to prevent partial builds.
-        let build = self
+        let temp_dir = self
             .build_context
             .cache()
-            .environment()
+            .build_dir()
             .map_err(Error::CacheWrite)?;
 
         // Build the wheel.
@@ -1748,13 +1748,13 @@ impl<'a, T: BuildContext> SourceDistributionBuilder<'a, T> {
             )
             .await
             .map_err(Error::Build)?
-            .wheel(build.path())
+            .wheel(temp_dir.path())
             .await
             .map_err(Error::Build)?;
 
         // Move the wheel to the cache.
         rename_with_retry(
-            build.path().join(&disk_filename),
+            temp_dir.path().join(&disk_filename),
             cache_shard.join(&disk_filename),
         )
         .await

crates/uv/src/commands/project/environment.rs

@@ -98,7 +98,7 @@ impl CachedEnvironment {
         }
 
         // Create the environment in the cache, then relocate it to its content-addressed location.
-        let temp_dir = cache.environment()?;
+        let temp_dir = cache.venv_dir()?;
         let venv = uv_virtualenv::create_venv(
             temp_dir.path(),
             interpreter,

crates/uv/src/commands/project/run.rs

@@ -323,7 +323,7 @@ pub(crate) async fn run(
             Some(environment.into_interpreter())
         } else {
             // Create a virtual environment.
-            temp_dir = cache.environment()?;
+            temp_dir = cache.venv_dir()?;
             let environment = uv_virtualenv::create_venv(
                 temp_dir.path(),
                 interpreter,
@@ -538,7 +538,7 @@ pub(crate) async fn run(
                 }
 
                 // Create a virtual environment
-                temp_dir = cache.environment()?;
+                temp_dir = cache.venv_dir()?;
                 uv_virtualenv::create_venv(
                     temp_dir.path(),
                     interpreter,
@@ -741,7 +741,7 @@ pub(crate) async fn run(
                 debug!("Creating isolated virtual environment");
 
                 // If we're isolating the environment, use an ephemeral virtual environment.
-                temp_dir = cache.environment()?;
+                temp_dir = cache.venv_dir()?;
                 let venv = uv_virtualenv::create_venv(
                     temp_dir.path(),
                     interpreter,
@@ -791,7 +791,7 @@ pub(crate) async fn run(
         Some(match spec.filter(|spec| !spec.is_empty()) {
             None => {
                 // Create a virtual environment
-                temp_dir = cache.environment()?;
+                temp_dir = cache.venv_dir()?;
                 uv_virtualenv::create_venv(
                     temp_dir.path(),
                     base_interpreter.clone(),