Replace the raw checksum with b64 decoded checksum for GCS

[ghost]

Fixes #5874

Replace the raw checksum with b64 decoded checksum for GCS

[codecov]

Codecov Report

Merging #5891 (68e2c72) into master (c773062) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

❗ Current head 68e2c72 differs from pull request most recent head 42bb525. Consider uploading reports for the commit 42bb525 to get more accurate results

@@            Coverage Diff             @@
##           master    #5891      +/-   ##
==========================================
- Coverage   88.83%   88.82%   -0.01%     
==========================================
  Files         162      162              
  Lines       11106    11108       +2     
  Branches     1805     1805              
==========================================
+ Hits         9866     9867       +1     
- Misses        959      960       +1     
  Partials      281      281              

Impacted Files	Coverage Δ
scrapy/mail.py	77.77% <0.00%> (-0.88%)	⬇️
scrapy/pipelines/files.py	72.00% <50.00%> (+0.09%)	⬆️

[Gallaecio]

Thanks!