add: PUEUE_CONFIG env variable support

[Nukesor]

Resolves #464

[github-actions]

Test Results

    3 files  ±0    22 suites  ±0   2m 53s ⏱️ -1s
144 tests +1  144 ✔️ +1  0 💤 ±0  0 ❌ ±0 
308 runs  +2  308 ✔️ +2  0 💤 ±0  0 ❌ ±0 

Results for commit 52c6881. ± Comparison against base commit 61fcfce.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

Merging #468 (52c6881) into development (94ecaaf) will increase coverage by 0.02%.
Report is 1 commits behind head on development.
The diff coverage is 71.42%.

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@@               Coverage Diff               @@
##           development     #468      +/-   ##
===============================================
+ Coverage        79.76%   79.79%   +0.02%     
===============================================
  Files               77       77              
  Lines             5511     5518       +7     
===============================================
+ Hits              4396     4403       +7     
  Misses            1115     1115              

Files	Coverage Δ
pueue/src/client/cli.rs	27.50% <ø> (ø)
pueue/src/daemon/cli.rs	33.33% <ø> (ø)
pueue_lib/src/settings.rs	72.89% <71.42%> (-0.05%)	⬇️

... and 2 files with indirect coverage changes