pgsql: Allow MSVC .bat wrappers to be called from anywhere

Allow MSVC .bat wrappers to be called from anywhere

Instead of using a hardcoded or default path to the perl file the .bat
file is a wrapper for, we use a path that means the file is found in
the same directory as the .bat file.

Patch by Anton Voloshin, slightly tweaked by me.

Backpatch to all live branches

Discussion: https://postgr.es/m/2b7a674b-5fb0-d264-75ef-ecc7a31e54f8@postgrespro.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c3879a7b4c87344d804483234a7278a68e8248d9

Modified Files
--------------
src/tools/msvc/build.bat     | 2 +-
src/tools/msvc/install.bat   | 2 +-
src/tools/msvc/pgbison.bat   | 2 +-
src/tools/msvc/pgflex.bat    | 2 +-
src/tools/msvc/vcregress.bat | 2 +-
5 files changed, 5 insertions(+), 5 deletions(-)