blob/all: Don't require SetIOFSCallback be called to use io/fs.FS functions

[vangent]

Fixes #3474.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.16%. Comparing base (d2adbc5) to head (777d6c7).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3479   +/-   ##
=======================================
  Coverage   73.16%   73.16%           
=======================================
  Files         113      113           
  Lines       14990    14984    -6     
=======================================
- Hits        10967    10963    -4     
+ Misses       3259     3258    -1     
+ Partials      764      763    -1     

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