Fixes to Windows prefix changes

[guilhermebs]

I have recently started observing that conda-unpack was not properly fixing prefixes of entry points in Windows packages installed with pip install ..
This is because conda-pack assumes that the placeholder is in lower-case. But by inspecting the entry point binary I found this not to be the case. This PR solves the issue by trying to fix the entry point using both lower-case and mixed-case forms of the placeholder.

environment:

ca-certificates           2021.7.5             haa95532_1
certifi                   2021.5.30        py39haa95532_0
openssl                   1.1.1l               h2bbff1b_0
pip                       21.2.4           py38haa95532_0
python                    3.9.6                h6244533_1
setuptools                52.0.0           py39haa95532_0
snake                     1.0                      pypi_0    pypi  <--- my package installed with `pip install .`, link: https://github.com/guilhermebs/CondaNSIS/tree/main/example/package
sqlite                    3.36.0               h2bbff1b_0
tzdata                    2021a                h5d7bf9c_0
vc                        14.2                 h21ff451_1
vs2015_runtime            14.27.29016          h5e58377_2
wheel                     0.37.0             pyhd3eb1b0_1
wincertstore              0.2              py39h2bbff1b_0

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[jezdez]

@anaconda-issue-bot check

[dbast]

Thanks!