DEPOT_TOOLS_WIN_TOOLCHAIN can be user-level envvar

DEPOT_TOOLS_WIN_TOOLCHAIN does not need to be a system variable; it can
be a user-level environment variable. Refer to DEPOT_TOOLS_WIN_TOOLCHAIN
as an "environment variable" rather than a "system variable".

Note: This seems to be originally added in
https: //codereview.chromium.org/2526563003.
Change-Id: Id0ffb6f779d7747f0dc78441ad32303c72593c21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3517704
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/main@{#980825}

docs/windows_build_instructions.md

@@ -93,7 +93,7 @@ Control Panel → System and Security → System → Search for "Edit environmen
 
 Add `C:\src\depot_tools` at the front. Note: If your system PATH has a Python in it, you will be out of luck.
 
-Also, add a DEPOT_TOOLS_WIN_TOOLCHAIN system variable in the same way, and set
+Also, add a DEPOT_TOOLS_WIN_TOOLCHAIN environment variable in the same way, and set
 it to 0. This tells depot_tools to use your locally installed version of Visual
 Studio (by default, depot_tools will try to use a google-internal version).