Small fixes to the build environment

These are 2 small commits which improves debugging and better separation with unit testing and GIMP-as-tool infrastructures.

Also unlike the branch name implies, there is no fix for the gimp-win-x64-cross job failures which we suddenly have (with no apparent changes on our side). I believe I handled the breakage with a fix in crossroad instead: https://gitlab.freedesktop.org/crossroad/crossroad/-/commit/c79ae45792028d7445348e86904c1c98f108ad6a

Let's try and if this MR succeeds, I'll merge.