fix: Escape tallies temporary file path properly (#4767)

### Description
#4755 broke the nightly build because I neglected to escape a string
literal properly. Works just fine as long as your directory separator is
`/`, but then Windows had to go and be all different. :)

### How has this been tested?
Existing tests with `run-deep-tests`.

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyCore/Compilers/CoverageInstrumenter.cs

@@ -64,7 +64,7 @@ public class CoverageInstrumenter {
  if (legend != null) {
  wr.Write("DafnyProfiling.CodeCoverage.Setup({0}", legend.Count);
  if (talliesFilePath != null) {
- wr.Write($", \"{talliesFilePath}\"");
+ wr.Write($", @\"{talliesFilePath}\"");
  }
  wr.Write(")");
  compiler.EndStmt(wr);