Add missing return scope to std.file

[dkorpel]

This blocks a fix for Issue 22977 - [dip1000] can escape scope pointer returned by nested function.

The offending code being:

void rmdirRecurse(scope const(char)[] pathname) @safe
{
    //No references to pathname will be kept after rmdirRecurse,
    //so the cast is safe
    rmdirRecurse(DirEntry((() @trusted => cast(string) pathname)()));
}

[dlang-bot]

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Bugzilla references

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8481"

[thewilsonator]

Failure is unrelated