pgsql: ci: Make runs-on overridable

ci: Make runs-on overridable

Previously what runners CI ran on was hardcoded. Unfortunately that
makes it harder to use faster or self hosted runners (e.g. in a private
repository where the runners are very slow, or self hosted runners for
cfbot).

Fix that by allowing to override the runner types via
repository/organization variables. How to do that is documented in
src/tools/ci/README.

Testing faster github hosted runners showed that we have some
dependencies on specific locations, fix most of those.

As part of this the "Determine enabled jobs" job moves to ubuntu-slim,
which the setup job already did. Normally this won't make a difference,
but seems more consistent.

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>
Co-authored-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/a2ejn7lfqolutzz7kozalbhy3bixdrujb4buc3pgbtlk4am2ba@wbv6v7riia33
Discussion: https://postgr.es/m/CAN55FZ1BcWBsWgCCk2GQkYN6tzDxCw2hVK2m3qeiwaNXUnjLVg@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/327f3b099960e1b5f47a47f2792eba20a4f12f18

Modified Files
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.github/workflows/pg-ci.yml | 72 ++++++++++++++++++++++++++++-----------------
src/tools/ci/README | 21 +++++++++++++
2 files changed, 66 insertions(+), 27 deletions(-)