extensions: replace execs with source

[sergio-costas]

Using source it is possible to simplify the chain, reduce memory usage and make execution faster.

This MR emulates "exec $@" using "source", thus avoiding to create a new process and launching a new shell each time a shell script must be executed from another shell script.

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DT-504

[codecov-commenter]

Codecov Report

Merging #3869 (bd22b75) into main (ed6f871) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head bd22b75 differs from pull request most recent head 16e7892. Consider uploading reports for the commit 16e7892 to get more accurate results

@@           Coverage Diff           @@
##             main    #3869   +/-   ##
=======================================
  Coverage   88.34%   88.35%           
=======================================
  Files          77       77           
  Lines        4666     4670    +4     
=======================================
+ Hits         4122     4126    +4     
  Misses        544      544           

Impacted Files	Coverage Δ
snapcraft/commands/names.py	89.83% <0.00%> (ø)
snapcraft/store/client.py	93.18% <0.00%> (+0.21%)	⬆️

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