rust: use rust-installer's way of disabling ldconfig

Instead of patching in a disable ldconfig argument, just use the way
exposed by the rust-installer scripts to disable running ldconfig.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>

recipes-devtools/rust/rust.inc

@@ -335,6 +335,9 @@ do_rust_arch_fixup[dirs] += "${WORKDIR}/mk-cfg"
 
 llvmdir = "${STAGING_DIR_NATIVE}/${prefix_native}"
 
+# prevent the rust-installer scripts from calling ldconfig
+export CFG_DISABLE_LDCONFIG="notempty"
+
 do_configure () {
     # Note: when we adjust the generated targets, rust doesn't rebuild (even
     # when it should), so for now we need to remove the build dir to keep