Fix PATH expansion in comment

[jtraglia]

I ran into an issue building blst on Windows via a GitHub CI runner:

./build.bat
The input line is too long.
The syntax of the command is incorrect.

With your help, we discovered that %PATH% was being expanded twice. First in the command & again in the comment. This caused the command to be approximately 2x the length of PATH. If the system were configured with a large PATH, this could cause the set PATH command to be longer than the 8192-character limit and, as a result, fail.

[dot-asm]

Thanks! As we looked into it yesterday, I've identified an additional potential problem related to msys interference. [And a typo.] Trouble is that it, msys, provides link.exe, which would be a problem if it gets ahead of MSVC linker on the PATH. Hence the suggestion is to close this and I'll commit a fix that addresses both problems [and the typo]...

[jtraglia]

Okay yeah that's totally fine. Will close this.