Xcode: POST_BUILD runs before signing, breaks builds on ARM Mac

With the Xcode generator on macOS arm64, the pattern

add_executable(myexe myexe.c)
add_custom_command(TARGET myexe POST_BUILD COMMAND myexe)

fails to build. The POST_BUILD command generates a "Run Script" build phase with a script that runs myexe, but Xcode's build system tries to run that phase before singing the executable.

The build fails with output:

Killed: 9  "/path/to/myexe"
Command PhaseScriptExecution failed with a nonzero exit code

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