ci/container: move rust build section into the script itself

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32241>

.gitlab-ci/container/build-rust.sh

@@ -5,6 +5,8 @@
 
 set -ex
 
+uncollapsed_section_start rust "Building Rust toolchain"
+
 # cargo (and rustup) wants to store stuff in $HOME/.cargo, and binaries in
 # $HOME/.cargo/bin.  Make bin a link to a public bin directory so the commands
 # are just available to all build jobs.
@@ -34,3 +36,5 @@ linker = "arm-linux-gnueabihf-gcc"
 [target.aarch64-unknown-linux-gnu]
 linker = "aarch64-linux-gnu-gcc"
 EOF
+
+section_end rust

.gitlab-ci/container/debian/test-base.sh

@@ -155,8 +155,6 @@ uncollapsed_section_switch wayland "Building Wayland"
 
 ############### Install Rust toolchain
 
-uncollapsed_section_switch rust "Installing Rust toolchain"
-
 . .gitlab-ci/container/build-rust.sh
 
 ############### Build Crosvm