Always install dxtbx without touching conda_base - redux

[ndevenish]

The original PR #474 went through a few variants - but was reverted in 2010053 due to issues in other downstream packages - see #473 (comment).

The problem; #473 - cctbx wants to be able to use a bootstrap-flavor dxtbx, but without writing to conda_base/, because it may be readonly or would otherwise prefer that it is not written to. The original intent of #474 was to fix this, and went through persistence of environment variable, to inverting the default, to just attempting to never write to conda_base in the first place.

This PR reintroduces the changes, with extra care to be taken to ensure that nothing in e.g. dials or xia2 is broken.

I believe that 0c0eeb2 actually fixes this - the dxtbx dispatchers are regenerated directly after configuration, where they are normally generated before configuration in the standard libtbx configuration process. The problem is that subsequent libtbx.refresh runs were apparently starting from scratch and not picking up the configuration change from libtbx.env.

[ndevenish]

So, I've run a fresh bootstrap with this - and the full set of dxtbx, dials, xia2 and the xfel_regression tests; only a single xia2 test failed - I'm looking into it but am not convinced that is related.

Edit: The xia2 test also fails without this change, so is something on my local computer/environment.

[codecov]

Codecov Report

Merging #511 (f3acbda) into main (b93bcfa) will not change coverage.
The diff coverage is n/a.

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##             main     #511   +/-   ##
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  Coverage   40.88%   40.88%           
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  Files         176      176           
  Lines       15495    15495           
  Branches     2786     2786           
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  Hits         6335     6335           
  Misses       8606     8606           
  Partials      554      554