cmake: Fix rundir installation accepting DESTDIR environment variable

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The environment variable DESTDIR is commonly used to specify an install-
time prefix for installation of build artifacts (e.g. for staging an
installation in a temporary directory while keeping the overall prefixes
intact).

This variable should be ignored by all targets but the install target,
which requires us to set it to an empty string/path when we use
`install` to set up our rundir.

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