Fix misuse of unbuffered os.Signal channel as argument to signal.Notify

[nvanbenschoten]

This addresses a new vet warning added in Go 1.17.

See https://golang.org/doc/go1.17#vet.

[codecov]

Codecov Report

Merging #63 (e86b0f4) into main (f6bdb73) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main     #63     +/-   ##
=======================================
+ Coverage   90.0%   90.1%   +0.1%     
=======================================
  Files         12      12             
  Lines       3471    3471             
=======================================
+ Hits        3124    3126      +2     
+ Misses       282     281      -1     
+ Partials      65      64      -1     

Impacted Files	Coverage Δ
internal/main.go	40.9% <100.0%> (ø)
stack/stack.go	85.2% <0.0%> (+0.8%)	⬆️

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[maruel]

Thanks!