refactor(tvix/derivation): pass fingerprint to build_store_path

All of these occurences are actually sha256 digests of some fingerprint
- which means there's not a lot of need to do the hashing outside.
Instead of passing in a digest, keep the sha256 hasher in the function,
and pass in a fingerprint.

This makes it much easier to construct fingerprints using format!() in
all consumers, because we need don't need to juggle with the hasher
anymore.

Change-Id: I2dc3af2cab6cf06f55ae6cbd9a8be95faf2a07b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7907
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/derivation/src/write.rs

@@ -16,9 +16,6 @@ pub const BRACKET_CLOSE: char = ']';
 pub const COMMA: char = ',';
 pub const QUOTE: char = '"';
 
-pub const COLON: &str = ":";
-pub const TEXT_COLON: &str = "text:";
-pub const SHA256_COLON: &str = "sha256:";
 pub const DOT_FILE_EXT: &str = ".drv";
 
 fn write_array_elements(