feat(exec): infer binary when only one binary is present

[pavelzw]

It would be cool if we could infer the binary being executed from conda-packages directly if only a single binary is present in $PREFIX/bin (how will this behave on windows?).

Then, something like this would work natively:

pixi exec output/osx-arm64/msgvault-0.6.5-ha05fd65_0.conda
# equivalent of
pixi exec -s output/osx-arm64/msgvault-0.6.5-ha05fd65_0.conda -- msgvault

pixi exec go-gum
# equivalent of
pixi exec -s go-gum -- gum