FIX #4409 : Use go os/user unless baremetal

[archie2x]

See #4409

src/os/user is currently taken from tinygo sources in all cases

This change:

• pulls in tinygo/src/os/user only when baremetal
• uses go version otherwise
• defines the osusergo build tag which uses a pure-go implementation of os/user
• separates the declaration of types and errors to user.go and (stubbed) implementations to unimplemented_user.go
• (re)imports GO1.22.6/src/os/user/user.go to tinygo/src/os/user/user.go

Should add stubbed user.{LookupId,LookupGroupId}() ?
Are there other cases where tinygo's version of this package should be used?

[aykevl]

Did you see #4324?

[archie2x]

Did you see #4324?

Nope! Doh!

[leongross]

thanks for working on this @aykevl!