keg: mkpath on main postgresql@X directories

[cho-m]

Fix logic in #16966 as it only matched subdirectories

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I didn't realize pattern missed top directory.

A stricter regex would be %r{^postgresql@\d+[$/]} but I doubt anything would use a directory name like postgresql@16<some-extra-text> and the change matches our other regex patterns (e.g. Python's /^python[23]\.\d+/)

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Example of problem with previous logic:

$ brew link temporal_tables
Linking /usr/local/Cellar/temporal_tables/1.2.2_1... 3 symlinks created.

$ realpath /usr/local/share/postgresql@16
/usr/local/Cellar/temporal_tables/1.2.2_1/share/postgresql@16

$ realpath /usr/local/share/postgresql@16/extension
/usr/local/Cellar/temporal_tables/1.2.2_1/share/postgresql@16/extension

[MikeMcQuaid]

Thanks again @cho-m!