Fixed cache warmup of paths which contain back-slashes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8781
| License       | MIT
| Doc PR        | n/a

From #8781

"
At this step in the warmup process there is a string replacement made, but paths with backslashes
are written into the cache file as something like `C:\\path\\to\\cache/pro_/...` but the str_replace
searches for the plain string `C:\path\to\cache/pro_/...`, which produces no matches.

The fix solves this by using var_export to escape the search (and replace) paths so they match the
format originally output into the cache file.
"