shiv generates invalid shebang line in git bash (windows/mingw)

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Hi, I'm having a strange issue where the interpreter that ends up in the shiv zipapp is different from what I give it with the `-p` option. I can't figure out where this translation is happening. I do this on Windows with "git bash", which is a mingw environment that ships with the standard git installer. Here are my steps, after setting up a venv and installing shiv:
```
/c/source/shiv$./env/Scripts/python --version
Python 3.6.8

/c/source/shiv$./env/Scripts/shiv --version
shiv, version 0.4.0

/c/source/shiv$uname -a
MINGW64_NT-10.0-14393 DEVHOST31 3.1.4-340.x86_64 2020-05-19 12:55 UTC x86_64 Msys

/c/source/shiv$./env/Scripts/shiv -e script:main -p '/usr/bin/env python' -r /dev/null -o out.pyz

/c/source/shiv$head -1 out.pyz
#!C:/Program Files/Git/usr/bin/env python

/c/source/shiv$./out.pyz
bash: ./out.pyz: C:/Program: bad interpreter: No such file or directory
```
Something is prefixing "C:/Program Files/Git/" to my provided interpreter, causing it to become invalid. Any help would be much appreciated!

The payload names no source file or test. Reproduce the command in Git Bash on Windows, then trace shebang generation for the -p option and the Windows/Mingw path translation. Done means the generated first line preserves the requested interpreter and the resulting zipapp runs.

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