Fix SUBPACKAGES make variable

[ulucinar]

Description of your changes

@turkenf has reported that the run make target is currently broken. Investigating the issue, the root cause is we do not compile the monolith binary because the SUBPACKAGES make variable is set to an empty string. This PR is an attempt to fix this issue.

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Tried running make run locally.

[turkenf]

• make run ran locally successfully:

  20:06:22 [ .. ] Running Crossplane locally out-of-cluster . . .
  UPBOUND_CONTEXT="local" /Users/fatihturken/Workspace/upbound/provider-gcp/_output/bin/darwin_arm64/monolith --debug --certs-dir=""

• After running make build.all publish BRANCH_NAME=main SUBPACKAGES="cloudplatform config":

  20:42:03 [ OK ] docker build build-e4b9b69b/provider-gcp-arm64
  20:42:03 [ .. ] Batch processing smaller provider packages for: cloudplatform config
  Pushing xpkg to index.docker.io/turkenf/provider-family-gcp:v1.3.0-rc.0.5.ga49d4e171.
  Pushing xpkg to index.docker.io/turkenf/provider-gcp-cloudplatform:v1.3.0-rc.0.5.ga49d4e171.
  xpkg pushed to index.docker.io/turkenf/provider-family-gcp:v1.3.0-rc.0.5.ga49d4e171
  xpkg pushed to index.docker.io/turkenf/provider-gcp-cloudplatform:v1.3.0-rc.0.5.ga49d4e171
  20:42:09 [ OK ] Done processing smaller provider packages for: cloudplatform config

  I think we need to handle XPKG_REG_ORGS like the same way.

• After running make build.all publish BRANCH_NAME=main:

  20:46:50 [ OK ] docker build build-e4b9b69b/provider-gcp-arm64
  20:46:51 [ .. ] Batch processing smaller provider packages for: monolith
  Pushing xpkg to index.docker.io/turkenf/provider-gcp:v1.3.0-rc.0.5.ga49d4e171.
  xpkg pushed to index.docker.io/turkenf/provider-gcp:v1.3.0-rc.0.5.ga49d4e171
  20:50:00 [ OK ] Done processing smaller provider packages for: monolith

[github-actions[bot]]

Successfully created backport PR #534 for release-1.2.