pgsql: ci: Make msys2 install smaller

ci: Make msys2 install smaller

We only needed git and diffutils installed via pacman because the already
installed tools were hidden by the login script resetting PATH. "Fix" that
instead by telling msys to leave the old PATH around.

Also don't install libbacktrace, it is not needed at the moment.

This commit is backpatched to 19 to keep CI between master and 19 the
same. Soon we'll backpatch CI support to regain coverage with lost with
cirrus-ci's shutdown.

Author: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/a2ejn7lfqolutzz7kozalbhy3bixdrujb4buc3pgbtlk4am2ba@wbv6v7riia33
Discussion: https://postgr.es/m/CAN55FZ1BcWBsWgCCk2GQkYN6tzDxCw2hVK2m3qeiwaNXUnjLVg@mail.gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f9c93b42e41a0ce0f9008576c7071e5d002857c0

Modified Files
--------------
.github/workflows/pg-ci.yml | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)