fix: allow using an env var in the signature.key_file field

[djgilcrease]

I want to use the same config file for local builds that are not signed and pipeline builds that are signed so I need to be able to specify the key_file as an env var.

[vercel]

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[codecov]

Codecov Report

Merging #282 (a68a03d) into master (e16b06a) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
+ Coverage   69.86%   70.00%   +0.13%     
==========================================
  Files           9        9              
  Lines        1085     1090       +5     
==========================================
+ Hits          758      763       +5     
  Misses        203      203              
  Partials      124      124              

Impacted Files	Coverage Δ
nfpm.go	86.00% <100.00%> (+0.73%)	⬆️

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.