ci: use ubuntu 20.04 for nightly builds on linux (#30787)

Ubuntu 22.04 has a newer glibc (2.34) which means builds
from there won't run on systems with older glibc, most
notably the wpt.fyi taskcluster runners which use 20.04
as the docker base image.

This is a temporary workaround until wpt upgrades to 22.04

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/workflows/linux.yml

@@ -61,7 +61,10 @@ env:
 jobs:
   build:
     name: Linux Build
-    runs-on: ubuntu-22.04
+    # Ubuntu 22.04 has glibc 2.34 so the binaries produced
+    # won't run on systems with older glibc e.g wpt.fyi
+    # runners which still use 20.04.
+    runs-on: ubuntu-${{ inputs.upload && '20.04' || '22.04' }}
     steps:
       - uses: actions/checkout@v3
         if: github.event_name != 'pull_request_target'