Once you have pkgsrc source and pkgsrc binary set up, chances are low
that one of them moves to a different location. And even if that's what
happens, you can simply rebuild pkg_check to update the PKGSRCDIR that
would be hardcoded in the pkg_chk binary.
For example, lintpkgsrc also hardcodes PKGSRCDIR during the build, plus
a good guess for LINTPKGSRC_MAKECONF. The latter has been hardcoded
since 2022, and I didn't receive any complaints about it.
Before 2015, pkglint also hardcoded PKGSRCDIR, but since it is obvious
from each pkglint invocation what to check and look for a nearby
../mk/bsd.pkg.mk, newer pkglint doesn't need this hardcoded variable
anymore.
Am I missing some point here why hardcoding PKGSRCDIR would be bad?