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_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9fd6d4eae3009658d0d941262f7dec5ca9b877c1

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(+)