src/poky:rust: Disable rust oe-selftest

After rust is upgraded to 1.71 and later versions, the rust oe-selftest gives
build errors due to unstable nightly options(see the error mentioned below). Thus, disable the test suite
until the issue is fixed

error: the option `Z` is only accepted on the nightly compiler
thread 'main' panicked at 'failed to gather the target spec for x86_64-poky-linux-gnu', synthetic_targets.rs:66:9

Following issues are created in Yocto bugzilla and rust upstream to track this issue-

https://bugzilla.yoctoproject.org/show_bug.cgi?id=15275
rust-lang/rust#115642
https://users.rust-lang.org/t/does-rust-test-suite-supports-nightly-options-during-bootstrapping-in-rust/103108
https://users.rust-lang.org/t/unable-to-read-target-specs-when-rust-1-73-is-bootstrapped-in-yocto-poky/102959

(From OE-Core rev: fa2d3cda8f7f932c0ed5c534c4bc820af2761cd5)

Signed-off-by: Yash Shinde <Yash.Shinde@windriver.com>
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: Yash Shinde