Writing temp file to os temp dir

[linzhp]

The directory passed to enumer may not be writable (e.g., when Bazel actions calls enumer). This PR makes it write to the OS default temp dir instead

[codecov-commenter]

Codecov Report

Merging #64 (a0342d7) into master (0f231a2) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   66.23%   66.23%           
=======================================
  Files           4        4           
  Lines         465      465           
=======================================
  Hits          308      308           
  Misses        146      146           
  Partials       11       11           

Impacted Files	Coverage Δ
stringer.go	61.04% <0.00%> (ø)

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