Init of msbuild env variables document

[f-alizada]

Fixes #9739

Context

Please see the description in attached issue

Changes Made

Added the new file in wiki, which will describe the behavior and usage of different environment variables that are being respected and used in MSBuild.

Testing

Nothing to test

Notes

Next steps would be to transfer all environment variables from https://github.com/dotnet/msbuild/blob/main/documentation/wiki/MSBuild-Tips-%26-Tricks.md to have a separate well indexed file for easy search.

[JanKrivanek]

Thank you for taking time to document this!!

It feels all the magic env vars should be documented on a single place - how about merging this info into

msbuild/documentation/wiki/MSBuild-Tips-&-Tricks.md

Line 15 in d68cc3a

# Environment Variables

or vice versa extracting (and linking) the Tips&Tricks Env Vars section to this doc?

[f-alizada]

Thank you for taking time to document this!!

It feels all the magic env vars should be documented on a single place - how about merging this info into

msbuild/documentation/wiki/MSBuild-Tips-&-Tricks.md

Line 15 in d68cc3a

# Environment Variables

or vice versa extracting (and linking) the Tips&Tricks Env Vars section to this doc?

Thank you for mentioning that! :) Actually I was planing to do that once current initiative with env variables in separate file is approved. As was mentioned in the PR description I suggest to link (as you mentioned) the MSBuild tips and tricks to this file, for proper indexing and easy search online, but of course this suggestion is open to discussion!

[JanKrivanek]

Oh I see now. Sounds good to me!

[AR-May]

Having MSBuild env variables documented in one place seems very reasonable to me for searching purposes.

[rainersigwald]

I'm not opposed to putting together a better list for our own purposes, but we need to be clear that most of the environment variables are intentionally undocumented and unsupported. They're there primarily as emergency escape hatches for users who get into bad problems, and we don't test the combinations and so on.