Ignore RUST_SRC_PATH if it is set to invalid value
Ignore RUST_SRC_PATH if it is set to invalid value

Folks report a ton of hard-to-diagnose issues, the solution for which
is "unset RUST_SRC_PATH". Let's just ignore RUST_SRC_PATH when it
won't work anyway!

rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer