ci: Drop mips/mips64 targets from CI

`mips-unknown-linux-gnu` and `mips64-unknown-linux-gnuabi64` have been
dropped to Tier 3 since rust 1.72 by rust-lang/compiler-team#648.
Hence, they are no longer available even in the nightly channel.

Signed-off-by: Manabu Sugimoto <[email protected]>

Author: ManaSugi

.github/workflows/build-test.yaml

@@ -96,8 +96,8 @@ jobs:
           - i686-unknown-linux-gnu
           - aarch64-unknown-linux-gnu
           - arm-unknown-linux-gnueabihf
-          - mips-unknown-linux-gnu
-          - mips64-unknown-linux-gnuabi64
+          #- mips-unknown-linux-gnu # It has been dropped to Tier 3 since 1.72
+          #- mips64-unknown-linux-gnuabi64 # It has been dropped to Tier 3 since 1.72
           - powerpc-unknown-linux-gnu
           - powerpc64-unknown-linux-gnu
           - riscv64gc-unknown-linux-gnu