[ci] Skip MIPS tests on the stable toolchain

MIPS targets are not available for the 1.72.1 toolchain. Prior to this
commit, attempting to roll the stable toolchain to 1.72.1 was failing
due to these missing targets: #447 (review)

This commit skips building/testing/etc MIPS targets with the stable
toolchain. This commit does not roll the stable toolchain. We want to
make sure that our auto-roll procedure works, and by only disabling
these target/toolchain pairs without rolling manually, we will give the
auto-roller a chance to try again within 24 hours (as opposed to having
to wait until the next stable toolchain release in up to six weeks).

.github/workflows/ci.yml

@@ -69,6 +69,12 @@ jobs:
             features: "--features __internal_use_only_features_that_work_on_stable"
           - crate: "zerocopy-derive"
             features: "--all-features"
+          # Exclude any combination in which the target is unavailable on
+          # the given toolchain.
+          - toolchain: "stable"
+            target: "mips-unknown-linux-gnu"
+          - toolchain: "stable"
+            target: "mips64-unknown-linux-gnuabi64"
 
     name: Build & Test (crate:${{ matrix.crate }}, toolchain:${{ matrix.toolchain }}, target:${{ matrix.target }}, features:${{ matrix.features }})