debian/gbp.conf: Use cargo-vendor-filterer from path during build

[3v1n0]

Ensure that gbp buildpackage works by using the locally-installed cargo-vendor-filterer, if any.

So trigger the package cleanup before the build with the proper PATH value, to ensure that we can have a properly working gbp buildpackage in all the versions we support.

As per this, building a clean source package is as easy as doing (or binaries can be built in a similar way):

gbp buildpackage -S

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.30%. Comparing base (36511cd) to head (58f1b5c).
Report is 574 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #859      +/-   ##
==========================================
- Coverage   83.43%   79.30%   -4.14%     
==========================================
  Files          83      104      +21     
  Lines        8689    10506    +1817     
  Branches       74       75       +1     
==========================================
+ Hits         7250     8332    +1082     
- Misses       1111     1707     +596     
- Partials      328      467     +139     

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