feat(tvix/glue): handle passAsFile

This extends derivation_to_build_request to handle passAsFile the same
way Nix does, and adds a unit test for it.

I opted to making this function fallible (if passAsFile contains a
non-existent env var), rather than pushing all of this into the
Derivation validate function.

Change-Id: I75b635f1f6c0c78d72b9a8fc7824f77e97b69951
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10522
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/glue/Cargo.toml

@@ -14,6 +14,7 @@ bytes = "1.4.0"
 tracing = "0.1.37"
 tokio = "1.28.0"
 thiserror = "1.0.38"
+sha2 = "0.10.8"
 
 [dependencies.wu-manber]
 git = "https://github.com/tvlfyi/wu-manber.git"