[Bug]: Windows install script is still flaky

[ayewo]

Describe the bug

Wanted to investigate the perf issues on Windows in #388 and #401 but had a bit of trouble getting the installer script to work on the Windows VM I used.

On the Windows Server 2019 VM I used for testing, the detection of a valid installation of python appeared to be a bit flaky.
In my testing:

Python

a. If you run the script for the first time, the check for a supported version of python will fail, even if a supported version was already installed.
b. If you re-run the script a second time, within the same PowerShell console, the check will pass and proceed to the check for the next dependency (i.e. git.)
c. If you open a fresh PowerShell console to re-run the script, it will fail to correctly detect that a supported version of python is installed (like in #a above).

Git

The detection of a valid installation of git was causing the install script to fail so I made some changes to make it a bit more robust.

The git install command somehow doesn't update the %PATH% to include the binaries for git and bash which is why the presence check always failed, even after the script downloaded & installed git.
To fix the issue: I refactored the code to explicitly add the path to those binaries to the USER and SYSTEM PATH.

OpenAdapt

I also made a few tentative changes (as per my proposal in #447):

• I moved the installation folder for OpenAdapt from the root drive at C:\OpenAdaptSetup\OpenAdapt to the user's home directory (at C:\Users\$USER\OpenAdapt) and added a check to delete it if it already exists. Right now when the script is re-run it will fail if the C:\OpenAdaptSetup\OpenAdapt folder exists and is non-empty.
• I also renamed a handful of variables to make them a bit clearer.

To Reproduce

See above.