Add julia_version = 1 conversion and run resolver with Base.julia_cmd

[JoshuaLampert]

I really want to get this done, such that this GitHub action is able to achieve the same results as v1 of the action. So I looked again at fixing julia-actions#37 (comment). If I understand correctly, there are two independent issues:

1. As stated in Unexpected behaviour when using julia_version: 1? julia-actions/julia-downgrade-compat#25, setting julia_version = 1 is currently interpreted as "any julia version 1.x.y". That's probably not what most people want when they set julia_version = 1. In this PR, I simply set it to VERSION. Alternatively, on could set it to the latest version.
2. Even with that resolved, as can be seen in my first example of Handle packages with local [sources] in test/Project.toml julia-actions/julia-downgrade-compat#37 (comment), we get ERROR: LoadError: could not find source path for package MbedTLS_jll when running with julia v1.10. This is caused by the fact the julia process, which runs Resolver.jl currently simply uses julia in the cmd, i.e., the latest available julia version (in this case julia v1.12). This leads to mismatches and thus the error (we can also see the reference to julia v1.12 in the stacktrace). Again, we have two options: Either we use the version, which was used to run the action or we use julia_version. Here, I opted for the first option by utilizing Base.julia_cmd().

This fixes julia-actions#25 and the problems described in julia-actions#37 (comment) (I still do not recover the same behavior as in v1, but I will explain that in the other PR).

[JoshuaLampert]

@ChrisRackauckas, could you take a look at this, please?