Environment variables no longer passed into build / how to detect Salsa CI from in the build?

Since Salsa CI switched to sbuild !569 (merged), environment variables (both the standard SALSA_CI* ones, and any that the user sets in salsa-ci.yml) are no longer passed into the build chroot, so are no longer available in debian/rules.

These packages are known to be affected:

~~pagure used to check CI_COMMIT_MESSAGE to avoid running all its tests on Salsa (which would time out).~~ Fixed
ceres-solver, eigen3, siconos set and check SALSABUILD to disable debug info on Salsa because they would otherwise be excessively large. @gladk
dh-ros checks for the standard SALSA_CI* to skip a version check that would fail. @roehling

Related to this: should there be a documented method to detect that you're on Salsa? One method that still works is to set a build profile in SALSA_DPKG_BUILDPACKAGE_ARGS, but I don't know whether there are better options.

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