Make parsing of vcvarsall.bat output more robust

Make parsing of vcvarsall.bat output more robust

Only consider variable names with alpanumeric characters, underscore
and parentheses. This avoids most problems when parsing output
containing multi-line variable values with arbitrary text.

In GitLab CI, one of the environment variables is CI_COMMIT_MESSAGE. In the commit immediately following this commit there is an URL containing an equal sign that caused the error below:

Set-Content : An object at the specified path
env:\https://docs.microsoft.com/en-us/visualstudio/ide/how-to-configure-projects-to-target-platforms?view does not
exist, or has been filtered by the -Include or -Exclude parameter.
At line:3 char:5
+     Set-Content "env:\$($matches[1])" $matches[2];
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (System.String[]:String[]) [Set-Content], Exception
    + FullyQualifiedErrorId : ItemNotFound,Microsoft.PowerShell.Commands.SetContentCommand