test: narrow subst pattern in non-existent-branch.t to avoid matching paths

[robinbb]

Summary

The output post-processing in
test/blackbox-tests/test-cases/pkg/pin-stanza/non-existent-branch.t
uses dune_cmd subst '-\d+' '-eol' to normalise the variable
end-column in the characters 6-<N>: part of dune's error
location. The pattern is too broad: any -<digits> substring in
the output matches, including path components in the sandbox root.

When the repo is checked out at a worktree whose directory contains
-<digits> (e.g. dune-pr-14116/), the substitution rewrites
that path component and BUILD_PATH_PREFIX_MAP stripping no
longer matches, so the test output contains a raw sandbox path and
the assertion fails.

Anchor the pattern to characters 6-\d+ so it only matches the
column range it was meant to normalise.

Test plan

• Reproduced the failure in a worktree checked out at a path
  containing -<digits> (e.g. dune-pr-14116/) on upstream/main.
• Applied the fix and verified the test passes in the same
  worktree path.
• Verified the test continues to pass in a worktree whose path
  does not match -<digits>.

[Copilot]

Pull request overview

This PR fixes a flaky blackbox test by narrowing an overly broad output normalization substitution so it no longer rewrites unrelated -<digits> substrings (notably in sandbox/worktree paths), which could prevent BUILD_PATH_PREFIX_MAP path stripping and cause assertion failures.

Changes:

• Tighten the dune_cmd subst regex from -\d+ to characters 6-\d+ in the test output post-processing.
• Update the replacement string accordingly to normalize the end-column range without affecting paths.

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