Allow wildcard "any" stack

[jkutner]

Summary

Fixes a bug that prevents * stacks from working.

Output

Before

$ pack build ...
...
ERROR: failed to build: validating stack mixins: buildpack heroku/procfile@0.7.0 does not support stack my_stack
make: *** [test] Error 1

After

$ pack build ...
...
Successfully built image python-cnb-test

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

[codecov]

Codecov Report

Merging #1234 (359df6c) into main (5bde87f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1234   +/-   ##
=======================================
  Coverage   80.95%   80.95%           
=======================================
  Files         138      138           
  Lines        8493     8493           
=======================================
  Hits         6875     6875           
  Misses       1180     1180           
  Partials      438      438           

Flag	Coverage Δ
os_linux	80.59% <100.00%> (ø)
os_macos	78.11% <100.00%> (ø)
os_windows	80.87% <100.00%> (ø)
unit	80.64% <100.00%> (ø)

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