Ignore build roots that contain '..'

[fthomas]

This should prevent exiting Scala Steward's workspace using build roots
that contain ...

[codecov]

Codecov Report

Merging #1991 (4c8a82b) into master (d3e6006) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1991   +/-   ##
=======================================
  Coverage   78.65%   78.66%           
=======================================
  Files         130      130           
  Lines        2258     2259    +1     
  Branches       52       57    +5     
=======================================
+ Hits         1776     1777    +1     
  Misses        482      482           

Impacted Files	Coverage Δ
.../org/scalasteward/core/repoconfig/RepoConfig.scala	100.00% <100.00%> (ø)

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