perf(tm2/bft/fail): use sync.Once instead of Getenv on each call

[thehowl]

minor change spotted when browsing tm2 code.

fail.Fail is called a lot in hot paths, like in the tm2 state machine. This avoids calling os.Getenv each time, which actually does a bunch of stuff, even when there is no associated env var.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.86%. Comparing base (5503cca) to head (78ed413).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
tm2/pkg/bft/fail/fail.go	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2805      +/-   ##
==========================================
+ Coverage   60.83%   60.86%   +0.03%     
==========================================
  Files         563      563              
  Lines       75193    75190       -3     
==========================================
+ Hits        45743    45765      +22     
+ Misses      26076    26058      -18     
+ Partials     3374     3367       -7     

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (ø)
gno.land	67.17% <ø> (ø)
gnovm	65.63% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	19.88% <ø> (-0.36%)	⬇️
tm2	62.08% <60.00%> (+0.19%)	⬆️

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