compress *.Rraw in release only, not git.

[mattdowle]

Follow up to #3933
Just adds the notes to CRAN_Release.cmd, plus some refinements to test.data.table() find the file with the .Rraw.bz2 extension. But other than that, base::sys.source() already accepts compressed input directly.

The saves another 0.75MB, to add to the saving in #3933 of 0.27MB brings the overall installed size of /inst/tests down from 1.7MB to 0.7MB. Plenty to avoid the 5.1MB breach of 5.0MB. We should be at around 4.0MB now even with a -g bloated .so.

[codecov]

Codecov Report

Merging #3937 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3937      +/-   ##
=========================================
+ Coverage    99.4%   99.4%   +<.01%     
=========================================
  Files          72      72              
  Lines       13642   13644       +2     
=========================================
+ Hits        13561   13563       +2     
  Misses         81      81

Impacted Files	Coverage Δ
R/test.data.table.R	100% <100%> (ø)	⬆️

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