Change linux release runner to use ubuntu-18.04 (#561)

ubuntu-latest uses Ubuntu 20.04, this causes issues with glibc as older versions of ubuntu/other distros use an older version.

This is fixed by building the release binary on `ubuntu-18.04`, which uses a version of glibc more widely available.

Ref: JohnnyMorganz/StyLua#444
JohnnyMorganz/StyLua#445

.github/workflows/release.yml

@@ -67,7 +67,7 @@ jobs:
         # -x86_64 to each release.
         include:
           - host: linux
-            os: ubuntu-latest
+            os: ubuntu-18.04
             target: x86_64-unknown-linux-gnu
             label: linux
 
@@ -150,4 +150,4 @@ jobs:
         uses: actions/upload-artifact@v3
         with:
           name: ${{ env.BIN }}-${{ env.PROJECT_VERSION }}-${{ matrix.label }}.zip
-          path: release.zip
+          path: release.zip