update install location & add systemwide option

[LilithHafner]

fixes #7

[codecov-commenter]

Codecov Report

Merging #12 (dbe90e6) into main (08ab856) will decrease coverage by 2.72%. The diff coverage is 81.63%.

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
- Coverage   84.37%   81.64%   -2.73%     
==========================================
  Files           1        1              
  Lines         128      158      +30     
==========================================
+ Hits          108      129      +21     
- Misses         20       29       +9     

Impacted Files	Coverage Δ
src/UpdateJulia.jl	81.64% <81.63%> (-2.73%)	:arrow_down:

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[LilithHafner]

This PR was complicated by the fact that install_location and default_install_location expand the scope of testing to include

• installing into a directory that doesn't exist
• linking to a bin that doesn't exist
• linking to a bin that isn't on path
• adding a directory with spaces as bin on windows

[LilithHafner]

The issue was passing an invalid set_default=true keyword argument on windows, we now gracefully ignore that argument with a warning.

[LilithHafner]

Need to document this behavior!