Fix nightly version for MIPS cross-compilation.

MIPS has moved to tier 3, we need the nightly version for
cross-compilation to this target. To avoid this, I reverted back to
1.74.1 (latest version with MIPS toolchain pre-compiled), however this
re-introduces #52. Currently the latest nightly version returns an error
during build:

 undefined reference to `pthread_getname_np'

Author: brocaar

Makefile

@@ -23,13 +23,13 @@ build-armv7-unknown-linux-musleabihf:
 
 build-mips-unknown-linux-musl:
 	# mips is a tier-3 target.
-	rustup +nightly component add rust-src
-	cross +nightly build -Z build-std=panic_abort,std --target mips-unknown-linux-musl --release --no-default-features --features semtech_udp
+	# rustup toolchain add nightly-2024-02-18-x86_64-unknown-linux-gnu
+	cross +nightly-2024-02-18 build -Z build-std=panic_abort,std --target mips-unknown-linux-musl --release --no-default-features --features semtech_udp
 
 build-mipsel-unknown-linux-musl:
 	# mipsel is a tier-3 target.
-	rustup +nightly component add rust-src
-	cross +nightly build -Z build-std=panic_abort,std --target mipsel-unknown-linux-musl --release --no-default-features --features semtech_udp
+	# rustup toolchain add nightly-2024-02-18-x86_64-unknown-linux-gnu
+	cross +nightly-2024-02-18 build -Z build-std=panic_abort,std --target mipsel-unknown-linux-musl --release --no-default-features --features semtech_udp
 
 # Build distributable binaries for all targets.
 dist: \