Run fish activation scripts with source instead of `.

[DonJayamanne]

For #7343

[codecov-io]

Codecov Report

Merging #7541 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7541      +/-   ##
==========================================
- Coverage   58.85%   58.84%   -0.01%     
==========================================
  Files         492      492              
  Lines       21823    21821       -2     
  Branches     3501     3500       -1     
==========================================
- Hits        12843    12841       -2     
  Misses       8179     8179              
  Partials      801      801

Impacted Files	Coverage Δ
...on/terminal/environmentActivationProviders/bash.ts	88.88% <100%> (-1.12%)	⬇️

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