Fix a JET warning

[fingolfin]

In the 'else' path, JET cannot prove that SCRATCH_DIR_OVERRIDE[] is (still) 'nothing', and thus reports a possible error. This then is also shown when analyzing packages that use Scratch. This can be resolved by assigning it first to a temporary variable.

[codecov[bot]]

Codecov Report

Merging #44 (90ebed6) into master (7df2dd3) will increase coverage by 0.11%. Report is 1 commits behind head on master. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   87.61%   87.73%   +0.11%     
==========================================
  Files           1        1              
  Lines         105      106       +1     
==========================================
+ Hits           92       93       +1     
  Misses         13       13              

Files	Coverage Δ
src/Scratch.jl	87.73% <66.66%> (+0.11%)	:arrow_up:

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[fingolfin]

Hmm, seems Scratch is broken on Julia nightly?

[fingolfin]

@staticfloat @fredrikekre @giordano any concerns about this PR? Otherwise I'd like to merge it and make a new release, to get my package "JET clean" ;-)

[giordano]

Looks good to me.

[staticfloat]

Looks fine to me; shall we add a JET pass to the CI in a follow-on PR so that we don't accidentally regress it?

[fingolfin]

@staticfloat I've added JET to the test suite, let me know if that's what you had in mind.