Run CI on Ubuntu 18.04 instead of latest version

.github/workflows/coverage.yml

@@ -6,7 +6,7 @@ on:
     branches: [master]
 jobs:
   codecov:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1

.github/workflows/deploy.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-18.04
             archive_file: texlab-x86_64-linux.tar.gz
             archive_cmd: tar -czvf {0} -C target/release texlab
             archive_type: application/gzip
@@ -40,7 +40,7 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
   ctan:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - run: git archive --prefix=texlab/ -o texlab.zip HEAD

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest, macos-latest]
+        os: [ubuntu-18.04, windows-latest, macos-latest]
     steps:
       - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1