ci: Use BuildJet Ubuntu 20.04 runners for better glibc compatibility (#18442)

Use BuildJet Ubuntu 20.04 runners.
- Linux arm64 unchanged (glibc >= 2.35)
- Linux x64 glibc requirement becomes to >= 2.31 (from glibc >= 2.35).

Note: Ubuntu 20.04 repo cmake (3.16.3) is normally too old to build Zed, but `ubuntu-2004` [includes cmake
3.30.3](https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2004-Readme.md#tools).

.github/workflows/release_nightly.yml

@@ -100,7 +100,7 @@ jobs:
     name: Create a Linux *.tar.gz bundle for x86
     if: github.repository_owner == 'zed-industries'
     runs-on:
-      - buildjet-16vcpu-ubuntu-2204
+      - buildjet-16vcpu-ubuntu-2004
     needs: tests
     env:
       DIGITALOCEAN_SPACES_ACCESS_KEY: ${{ secrets.DIGITALOCEAN_SPACES_ACCESS_KEY }}
@@ -117,7 +117,7 @@ jobs:
         run: echo "$HOME/.cargo/bin" >> $GITHUB_PATH
 
       - name: Install Linux dependencies
-        run: ./script/linux
+        run: ./script/linux && ./script/install-mold 2.34.0
 
       - name: Limit target directory size
         run: script/clear-target-dir-if-larger-than 100