[release-1.18] fix: return better error when using pack+python

[knative-prow-robot]

This is an automated cherry-pick of #2904

[matejvasek]

Probably best we can do for 1.18 is better error message. Pack simple won't work with python there.

[matejvasek]

Also many test will probably fail because the test code is not updated for the ever changing outer word.

[knative-prow]

@knative-prow-robot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
unit-tests_func_release-1.18	f801c13	link	true	/test unit-tests

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.66%. Comparing base (c1160e1) to head (f801c13).

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                @@
##           release-1.18    #3090      +/-   ##
================================================
+ Coverage         48.42%   48.66%   +0.23%     
================================================
  Files               129      131       +2     
  Lines             15787    15799      +12     
================================================
+ Hits               7645     7688      +43     
+ Misses             7455     7425      -30     
+ Partials            687      686       -1     

Flag	Coverage Δ
unit-tests	48.66% <0.00%> (?)

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[gauron99]

yeah that looks reasonable to me

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gauron99, knative-prow-robot

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[gauron99]

/hold for @lkingland input