lang: Remove a potential panic while getting the IDL in declare_program!

[acheroncrypto]

Problem

A part of the getting the IDL process can panic if getting the environment variable fails:

anchor/lang/attribute/program/src/declare_program/mod.rs

Line 37 in 6ff6655

let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").expect("Failed to get manifest dir");

Although this is unlikely to panic given this is a cargo environment variable, it still makes sense to convert this to an Err variant rather than potentially panicking, especially considering the get_idl function already returning a Result type.

Summary of changes

Remove a potential panic while getting the IDL in declare_program!.

[vercel]

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