Upload nightly builds to Github Releases

This change extends the `mach upload-nightly` command to
publish the nightly builds for all platforms as GH Release
assets.

The GH releases are made on a separate repository so
that we can persist older nightly builds without having
to accumulate git tags for them.

Some design tradeoffs in this approach are:
1. To allow the 'latest' link from servo.org to remain stable,
the release assets are named 'servo-latest.{ext}' instead of
containing the release tag/date.
2. The release is created as draft and published atomically
when all platforms have been built successfully. This allows
us to link to the 'latest' alias from servo.org while
gauranteeing that it contains builds for all platforms.
The other option here would be to have code in servo.org UI
that uses GH API to find the most recent release with a
successful build for a given platform.
3. The tags in the nightly repo are all based on the same
commit that has no relation to servo code base.

Signed-off-by: Mukilan Thiyagarajan <me@mukilan.in>

.github/workflows/linux.yml

@@ -16,6 +16,9 @@ on:
         required: false
         default: false
         type: boolean
+      github-release-id:
+        required: false
+        type: string
   workflow_dispatch:
     inputs:
       layout:
@@ -82,9 +85,14 @@ jobs:
           path: target/release/servo-tech-demo.tar.gz
       - name: Upload
         if: ${{ inputs.upload }}
-        run: python3 ./mach upload-nightly ${{ env.PACKAGE }} --secret-from-environment
+        run: |
+          python3 ./mach upload-nightly ${{ env.PACKAGE }} \
+            --secret-from-environment \
+            --github-release-id ${{ inputs.github-release-id }}
         env:
           S3_UPLOAD_CREDENTIALS: ${{ secrets.S3_UPLOAD_CREDENTIALS }}
+          NIGHTLY_REPO_TOKEN: ${{ secrets.NIGHTLY_REPO_TOKEN }}
+          NIGHTLY_REPO: ${{ github.repository_owner }}/servo-nightly-builds
       - name: Package binary
         run: tar -czf target.tar.gz target/release/servo resources
       - name: Archive binary