chore(brioche-packed): prefer usage of core:: instead of std::

[jaudiger]

From #88 (review), this PR enables the lint clippy:std_instead_of_core in two crates:

• brioche-packed-plain-exec
• brioche-packed-userland-exec

It ensures to use structures from the core:: namespace over the std:: namespace when it's possible, to latter help to transition to #![no_std].

clippy:std_instead_of_core cannot be set directly in the Cargo.toml of both crates since we are inherited from the workspace Clippy configuration, and it cannot be overridden from the Cargo.toml file:

[lints]
workspace = true