std.Build: resolved generated paths are cwd-relative

The doc comment here agreed with the implementation, but not with *any*
`Step` which populates a `GeneratedFile`, where they are treated as
cwd-relative. This is the obvious correct choice, because these paths
usually come from joining onto a cache root, and those are cwd-relative
if not absolute.

This was a pre-existing bug, but #23836 caused it to trigger more often,
because the compiler now commonly passes the local cache directory to
the build runner process as a relative path where it was previously an
absolute path.

Resolves: #23954

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