fix: return better error when using pack+python

[matejvasek]

No description provided.

[lkingland]

/approve
/lgtm

Much better UX

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lkingland, matejvasek

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 61.51%. Comparing base (58fb81a) to head (d5a60ea).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/builders/buildpacks/builder.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2904      +/-   ##
==========================================
+ Coverage   60.75%   61.51%   +0.75%     
==========================================
  Files         129      129              
  Lines       16327    16330       +3     
==========================================
+ Hits         9920    10045     +125     
+ Misses       5479     5327     -152     
- Partials      928      958      +30     

Flag	Coverage Δ
e2e-tests	42.39% <0.00%> (+1.39%)	⬆️
integration-tests	47.99% <0.00%> (+1.72%)	⬆️
unit-tests	48.12% <0.00%> (-0.01%)	⬇️

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[matejvasek]

/cherry-pick release-1.18

[knative-prow-robot]

@matejvasek: new pull request created: #3090

In response to this:

/cherry-pick release-1.18

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