Remove target locking

[idelvall]

This PR removes the locking in target mount, and also introduces a single target mount as opposed to one per target as we had before.

• Target locking was introduced for two reasons:
  • To make our remove fingerprint mode to work, without affecting other builds
  • To make targets atomic, in the sense that no other ongoing build could interfere with the copied artifacts.
• Multiple targets were used to enable build parallelism (of different targets)

This PR removes the fingerprinting deletion code, (now --keep-ts will be required in COPY commands), and the responsibility of ensuring that the artifacts of concurrent builds do not overwrite each other is left to the user.

This results in a simpler API, and a reduced cache usage

[idelvall]

I was assuming that Cargo locking was more fine gained (for target) than it apparently it is: https://github.com/rust-lang/cargo/pull/2486. Looks like target folder is locked globally so, we definitely don't want such limitation in Earthly builds.