fix: environment activation for windows

[ruben-arts]

Closes prefix-dev/pixi#437

The activation runs a this openssl_activate.bat when building the environment:

@echo off
if "%SSL_CERT_FILE%"=="" (
    set SSL_CERT_FILE=%CONDA_PREFIX%\Library\ssl\cacert.pem
    set __CONDA_OPENSLL_CERT_FILE_SET="1"
)

Which results in the following result when we call the env in cmdexe:

__CONDA_OPENSLL_CERT_FILE_SET="1"

This seems to be the only shell that does it that way. But our parser just keeps those " around which breaks because we add them again in our powershell fn set_env_var

This PR strips the ".

[baszalmstra]

Very nice find!