[rust] Lock rust version to specific nightly

We require a nightly build, but this makes CI fragile to breakage in the
nightly release.  Lock this to the latest known working version. This
will have to be periodically revisited or as new features might be
needed.

Change-Id: I9d00fbe74d724402f7b3909d4fecba2d27d81d59

Author: d3zd3z

lib/rust_support/rules.mk

@@ -21,6 +21,7 @@ MODULE_SRCDIR := $(MODULE)
 MODULE_BUILDDIR := $(call TOBUILDDIR,$(MODULE_SRCDIR))
 
 MODULE_SRCS := \
+	       $(MODULE_SRCDIR)/rust-toolchain.toml \
 	       $(MODULE_SRCDIR)/Cargo.toml \
 	       $(MODULE_SRCDIR)/src/lib.rs
 MODULE_OBJS := $(foreach d,$(MODULE_SRCS),$(call TOBUILDDIR,$(d)))
@@ -62,7 +63,7 @@ $(MODULE_OBJECT): MODULE_BUILDDIR:=$(MODULE_BUILDDIR)
 # Override with module local values for the build rule.
 $(MODULE_OBJECT): $(MODULE_OBJECT).phony $(MODULE_OBJS) $(CARGO_CONFIG)
 	cd $(MODULE_BUILDDIR); \
-		cargo +nightly build
+		cargo build
 
 EXTRA_OBJS := $(EXTRA_OBJS) $(MODULE_OBJECT)
 

lib/rust_support/rust-toolchain.toml.in

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "nightly-2025-12-04"
+components = ["rust-src"]