`\$` is interpreted before being passed to `re.search()`, but luckily
for us the escape is also invalid and because of that, python 3.12+
warns us about it.
Use a raw string instead, so that the `\` is passed untouched to
`re.search()`.
Fixes: aa04b47c6e ("intel/perf: add support for GtSlice/GtSliceXDualsubsliceY variables")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26355>
Eric Engestrom
1942073112