Use posix paths in devenv's PATH variable

[DFOVIT]

Currently the devenv dumped for vscode on windows contains windows paths :
PATH="C:\dev\tools\meson\test cases\unit\90 devenv\builddir\;C:\dev\tools\meson\test cases\unit\90 devenv\builddir\subprojects\sub"
which will fail when a directory start with a n for example : c:\this\will\not\work.

When using posix paths, vscode is happy.

• Before patch :

dump sh/export: PATH="C:\dev\tools\meson\test cases\unit\90 devenv\builddir\;C:\dev\tools\meson\test cases\unit\90 devenv\builddir\subprojects\sub;$PATH"
dump vscode: PATH="C:\dev\tools\meson\test cases\unit\90 devenv\builddir\;C:\dev\tools\meson\test cases\unit\90 devenv\builddir\subprojects\sub"

• After patch :

dump sh/export: PATH="C:/dev/tools/meson/test cases/unit/90 devenv/builddir;C:\dev\tools\meson\test cases\unit\90 devenv\builddir\subprojects\sub;$PATH"
dump vscode: PATH="C:/dev/tools/meson/test cases/unit/90 devenv/builddir;C:\dev\tools\meson\test cases\unit\90 devenv\builddir\subprojects\sub"

This is a subset that @amyspark kindly agreed to add to #12194

[xclaesse]

That's not the only place we have paths, they'll all have to be fixed then... But that sounds like a vscode issue, they should not assume that env file is escaped. Or we could also replace('\\', '\\\\') in the whole file since we already have vscode specific dump format.