cmd/go: fix coverage overlay

[kumarlokesh]

cmd/go: fix coverage with -overlay and -coverpkg

The issue occurred when using both -overlay and -coverpkg flags together.
The coverage tool was being called with original file paths instead of
the overlaid file paths, causing incorrect coverage instrumentation.

The fix modifies the cover method in cmd/go/internal/work/exec.go to:

1. Properly handle overlay mapping by using fsys.Actual() to get the
   actual file paths from the overlay filesystem
2. Pass these overlaid file paths to the coverage tool

Test has been added to verify this behavior. The test:

1. Creates an overlay that modifies function behavior to panic
2. Verifies that tests run with the overlay use the overlaid versions
3. Confirms that coverage instrumentation works correctly with the overlay

Fixes #73802

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Lokesh Kumar:

Patch Set 1:

(1 comment)

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This PR (HEAD: 7717182) has been imported to Gerrit for code review.

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