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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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a71f55652276805289d73dd88bc8f18e4a4d9ab2

Modified Files
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src/tools/msvc/install.pl   | 4 ++++
src/tools/msvc/mkvcbuild.pl | 4 ++++
src/tools/msvc/vcregress.pl | 2 ++
3 files changed, 10 insertions(+)