hadrian: Always canonicalize topDirectory
Hadrian's `topDirectory` is intended to provide an absolute path to the root of the GHC tree. However, if the tree is reached via a symlink this One question here is whether the `canonicalizePath` call is expensive enough to warrant caching. In a quick microbenchmark I observed that `canonicalizePath "."` takes around 10us per call; this seems sufficiently low not to worry. Alternatively, another approach here would have been to rather move the canonicalization into `m4/fp_find_root.m4`. This would have avoided repeated canonicalization but sadly path canonicalization is a hard problem in POSIX shell. Addresses #22451. (cherry picked from commit 5efa9ca5)
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