fix(ci): grant contents:read so desktop builds can clone the internal fork

The reusable build jobs ran with permissions: {} (fine for upstream's public
repo, which checks out with no token). This fork is internal, so checkout failed
with 'Repository not found'. Grant contents:read on the linux/macos/windows
caller jobs and the reusable build jobs, mirroring the prepare job.

.github/workflows/build_desktop_linux.yaml

@@ -76,6 +76,9 @@ jobs:
         # We build on native infrastructure as matrix-seshat fails to cross-compile properly
         # https://github.com/matrix-org/seshat/issues/135
         runs-on: ${{ inputs.runs-on || (inputs.arch == 'arm64' && 'ubuntu-22.04-arm' || 'ubuntu-22.04') }}
+        # contents:read so the checkout can clone the fork when it is a private/internal repo
+        permissions:
+            contents: read
         env:
             HAK_DOCKER_IMAGE: ghcr.io/element-hq/element-web/desktop-build-env:${{ case(github.event_name == 'push', inputs.ref || github.ref_name, github.event_name == 'release', 'staging', 'develop') }}
         steps: