fix: drop -Zpanic-on-unsupported from miri invocation

The flag was removed in
rust-lang/miri#3952.

This flag was mostly useful when running miri on a codebase for the
first time, in combination with --no-fail-fast, to quickyl determine
which tests could actually be ran under miri (as without this option,
miri would abort the entire runtime on the first unknown FFI call,
ignoring --no-fail-fast, so one could only determine these tests one at
a time). However, for continuous integration, it didn't actually have
any value, so we're not loosing anything by removing it.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

.buildkite/custom-tests.json

@@ -22,7 +22,7 @@
     },
     {
       "test_name": "miri",
-      "command": "RUST_BACKTRACE=1 MIRIFLAGS='-Zmiri-disable-isolation -Zmiri-panic-on-unsupported -Zmiri-backtrace=full' cargo +nightly miri test --features backend-mmap",
+      "command": "RUST_BACKTRACE=1 MIRIFLAGS='-Zmiri-disable-isolation -Zmiri-backtrace=full' cargo +nightly miri test --features backend-mmap",
       "platform": ["x86_64", "aarch64"]
     },
     {