pgsql: Fix perl searchpath for modern perl for MSVC tools

Fix perl searchpath for modern perl for MSVC tools

Modern versions of perl no longer include the current directory in the
perl searchpath, as it's insecure. Instead of adding the current
directory, we get around the problem by adding the directory where the
script lives.

Problem noted by Victor Wagner.

Solution adapted from buildfarm client code.

Backpatch to all live versions.

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/075641fd001bf2b59ac452fb645aecb4252bb860

Modified Files
--------------
src/tools/msvc/install.pl | 4 ++++
src/tools/msvc/mkvcbuild.pl | 4 ++++
src/tools/msvc/vcregress.pl | 3 +++
3 files changed, 11 insertions(+)