install: Use read_file from composefs-boot

[cgwalters]

Just reducing code here.

[gemini-code-assist]

Code Review

This pull request is a good simplification. It removes a local implementation of read_file and replaces it with the one from the composefs-boot crate, which helps reduce code duplication. The related changes to imports are also correct. Overall, this is a solid refactoring.

[jeckersb]

LGTM, with note that make validate-rust is failing in the composefs-backend branch unrelated to this change:

(cd crates/lib && cargo check --no-default-features)

[... snip ...]

error[E0433]: failed to resolve: could not find `parsers` in the crate root
  --> crates/lib/src/deploy.rs:32:12
   |
32 | use crate::parsers::bls_config::{parse_bls_config, BLSConfig};
   |            ^^^^^^^ could not find `parsers` in the crate root
   |
note: found an item that was configured out
  --> crates/lib/src/lib.rs:41:16
   |
41 | pub(crate) mod parsers;
   |                ^^^^^^^
note: the item is gated behind the `grub` feature
  --> crates/lib/src/lib.rs:40:7
   |
40 | #[cfg(feature = "grub")]
   |       ^^^^^^^^^^^^^^^^

[cgwalters]

Yeah it'd probably be messy right now to gate it via a feature, but...the grub stuff is super messy itself and it would be nice to cordon it off a bit in the future...

[cgwalters]

LGTM, with note that make validate-rust is failing in the composefs-backend branch unrelated to this change:

Fixed in #1512