cargo.bbclass: ensure extra RUSTFLAGS are properly passed

The RUSTFLAGS env var overrides the rustflags specified in the cargo
config so we need to take anything passed in the environment variable
and put it in the config.

recipes-devtools/cargo/cargo.inc

@@ -67,6 +67,10 @@ SRC_URI = "\
 
 B = "${S}"
 
+# Can't use the default from rust-vars as it cause a compiler crash
+# the crate_hash option will cause the compiler to crash
+RUSTFLAGS = "-C rpath ${RUSTLIB}"
+
 PACKAGECONFIG ??= ""
 
 # Note: this does not appear to work very well due to our use of bitbake triples