R PACKAGES file lost on upgrade to 3.20.x

When upgrading to Nexus Repository Manager (NXRM) 3.20.x from earlier versions, the R PACKAGES file is lost/unavailable on upgrade.

To reproduce:

1. On a < 3.20 NXRM version e.g. 3.18 install the R-Repository plugin (https://repo1.maven.org/maven2/org/sonatype/nexus/plugins/nexus-repository-r/)
2. Configure a hosted R repository and upload a package (note - proxy repos are also affected).
3. Attempt to download the PACKAGES.gz file and confirm that it exists e.g. http://localhost:8081/repository/r-hosted/src/contrib/PACKAGES.gz
4. Upgrade to 3.20.x
5. Post upgrade, attempt to access the PACKAGES file.

Expected: src/contrib/PACKAGES.gz is available and can be downloaded.

Actual: request to src/contrib/PACKAGES.gz returns a 404.

Workaround: Uploading a new package after the upgrade should trigger the PACKAGES file to be recreated. The following should be logged in the nexus.log after the upload:

INFO [event-6-thread-31] admin org.sonatype.nexus.repository.r.internal.hosted.RPackagesBuilderFacetImpl - Rebuilding R PACKAGES.gz metadata for repository r-hosted