Bottle build may fail if target branch receives commits

[scpeters]

The generic-release-homebrew_triggered_bottle_builder job builds bottles for separate configurations with jobs that run separately. Currently, the first build is for osx_catalina, followed by osx_bigsur. Each configuration job attempts to check out refs/remotes/origin/pr/*/merge for the corresponding pull request.

If there is a commit to the target branch after the first configuration has started and before the second, the second job may fail with an error message like the following:

Checking out Revision dc6001d45a119903288bc3280342d90083d8fefd (refs/remotes/origin/pr/1987/merge)
 > git config core.sparsecheckout # timeout=10
 > git checkout -f dc6001d45a119903288bc3280342d90083d8fefd # timeout=10
FATAL: Could not checkout dc6001d45a119903288bc3280342d90083d8fefd
hudson.plugins.git.GitException: Command "git checkout -f dc6001d45a119903288bc3280342d90083d8fefd" returned status code 128:
stdout: 
stderr: fatal: reference is not a tree: dc6001d45a119903288bc3280342d90083d8fefd

	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:2671)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$1100(CliGitAPIImpl.java:84)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$9.execute(CliGitAPIImpl.java:2989)

https://build.osrfoundation.org/job/generic-release-homebrew_triggered_bottle_builder/901/