ci(build_all): keep build artifacts for 1 day

Every build_all_targets run uploads about 1.3 GB of firmware, and
without retention-days it took the 90-day repository default. At
1,200-1,500 runs a month that held roughly 10 TB of Actions storage,
about $2k/month gross. PR artifacts only exist so authors can download
a build to test, and on pushes and tags the artifacts job copies them
to S3 and the GitHub Release in the same run.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/build_all_targets.yml

@@ -258,6 +258,9 @@ jobs:
         with:
           name: px4_${{matrix.group}}_build_artifacts
           path: artifacts/
+          # Long enough for PR authors to download a build, and for the
+          # artifacts job below to copy push and tag builds to S3.
+          retention-days: 1
 
       - uses: ./.github/actions/save-ccache
         if: success()