build-configs: move rust builds under mainline cfg

PR kernelci#1468 added the mainline rust build in a separate
`rust` build configuration instead of adding to the
already existing mainline config which can cause
unnecessary builds. Move it in the proper place.

Suggested-by: Guillaume Tucker <[email protected]>
Signed-off-by: Adrian Ratiu <[email protected]>

Author: 10ne1

config/core/build-configs.yaml

@@ -834,6 +834,11 @@ build_configs:
       clang-15:
         build_environment: clang-15
         architectures: *arch_clang_configs
+      rustc-1.62:
+        build_environment: rustc-1.62
+        fragments: [rust, rust-samples, kselftest]
+        architectures:
+          x86_64: *x86_64_arch
 
   matthiasbgg-for-next:
     tree: matthiasbgg
@@ -1007,16 +1012,6 @@ build_configs:
     branch: 'v5.15-rt'
     variants: *preempt_rt_variants
 
-  rust:
-    tree: mainline
-    branch: 'master'
-    variants:
-      rustc-1.62:
-        build_environment: rustc-1.62
-        fragments: [rust, rust-samples, kselftest]
-        architectures:
-          x86_64: *x86_64_arch
-
   rust-for-linux:
     tree: rust-for-linux
     branch: 'rust'