Only run CI rebase logic on PRs

[conorsch]

Status

Ready for review.

Description of Changes

We recently (#3617) modified the CI rebase logic to accommodate more
than just the develop branch. That's good! However, the new logic
assumed that the env var CIRCLE_PULL_REQUEST would be defined. Turns
out it's only defined on PRs, which makes sense, but not during CI runs
against a base branch such as "develop".

Modified the rebase script to execute the main block if-and-only-if
CIRCLE_PULL_REQUEST is set. Additionally, sprinkled in some more error
checking bash settings at the top of the file, to make future debugging
sessions more straightforward (set -u in particular makes mistakes
like this quite clear).

Closes #3639.

Testing

As we discovered in in #3617, we'll need to confirm this works after merge, as well as at the PR phase. Review the logic and double-check the error-checking for sanity. I SSH'd into the failed build in order to debug interactively.

Deployment

None, CI only.

Checklist

If you made changes to the server application code:

• Linting (make ci-lint) and tests (make -C securedrop test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

If you made changes to documentation:

• Doc linting (make docs-lint) passed locally

[conorsch]

@redshiftzero Testing here is a bit tricky. Try pulling up one of the old failed builds in CircleCI, specifically one of the develop ones—not a PR, since the logic doesn't fail there—and re-run it with SSH. Copy/paste in the script as presented here, execute it, and confirm you exit zero. (It should skip all the logic, which you can observe if you set -x at the top of the script.)

[redshiftzero]

SSHed into CI box on develop to test, logic worked as expected. Thanks!

[codecov-io]

Codecov Report

Merging #3641 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #3641   +/-   ##
========================================
  Coverage    85.04%   85.04%           
========================================
  Files           37       37           
  Lines         2367     2367           
  Branches       260      260           
========================================
  Hits          2013     2013           
  Misses         290      290           
  Partials        64       64

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