Disable Python detector's use of PYTHON_VERSION

[briandealwis]

Fixes: #3918

Description

The Python language runtime detection for skaffold debug was blindly accepting any image with the PYTHON_VERSION environment variable set. Although this does identify an Python-based image, successfully transforming the container requires that the image's entrypoint or command actually invoke the python interpreter. So the presence of PYTHON_VERSION does not necessarily mean that the image can be transformed

User facing changes (remove if N/A)

Should avoid a possible panic.

Follow-up Work (remove if N/A)

I need to rework the Apply interface to remove the possibility of this nil panic.

@code128: if you are willing to try this on your insta-puller, you can build a private copy of skaffold by running go build -o skaffold ./cmd/skaffold/. Skaffold does require Go 1.14 for building.

[codecov]

Codecov Report

Merging #3919 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
pkg/skaffold/debug/transform_python.go	84.70% <ø> (-0.36%)	⬇️