Make Bash path configurable

[pditommaso]

This PR allows the customisation of Bash interpreter used by Nextflow go a user provided path defined via nextflow config file or an environment variable (to be done).

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[rollf]

Thank you for addressing my issues @pditommaso.

NixOS currently patches Nextflow so that it can run there even though /bin/bash is not available. This works great unless one wants to enable tracing. Instead of improving the patch, I thought it might be nice to solve the issue upstream. Hence my motivation for #5432. I closed that PR in favor of #5684 (because the latter is more clean and I also updated my initial implementation since #4209 got merged in the meantime).

Now, #5684 got also closed and my approach will not be merged (reasoning is mentioned here) in favor of this PR here. I believe this PR here does not improve the situation for NixOS (see my reasoning here and here).

Thus, I see two options now

• Either: I'm wrong and this PR here does indeed improve the situation for NixOS in all 4 scenarios (docker on/off x tracing on/off) with minimal additional patching/configuration upfront but without having to touch the actual pipeline and without having to change process.shell when enabling/disabling docker/tracing.
• Or: This PR here does not help and I thus suggest to not merge if it was only motivated by supporting NixOS (since Nextflow doesn't need it). The Nix package definition for Nextflow will then continue to patch vanilla Nextflow; the patch will of course be improved to allow the tracing scenario (i.e. simply derive a patch from #5684 and apply it).

I can test option 1) but would need advice how you envision this to be used. However, we might also directly agree on option 2).

(For option 2, it would make my life easier if this PR here does not get merged. But this is of course entirely up to others to decide)

[rollf]

@pditommaso could you have a look at my comment above and give some feedback? Thanks.