build/toolchain: remove deprecated use of @bazel_tools

@bazel_tools//platforms will be removed with bazel 6.
https://github.com/bazelbuild/bazel/issues/8622

Change-Id: Iee484f68d76d83ec6d874d0eb4457bb273dc5ed8
Reviewed-on: https://review.monogon.dev/c/monogon/+/1703
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/build/toolchain/BUILD.bazel b/build/toolchain/BUILD.bazel
index 4a12064..84546b7 100644
--- a/build/toolchain/BUILD.bazel
+++ b/build/toolchain/BUILD.bazel
@@ -51,12 +51,12 @@
 toolchain(
     name = "host_cc_toolchain",
     exec_compatible_with = [
-        "@bazel_tools//platforms:linux",
-        "@bazel_tools//platforms:x86_64",
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
     ],
     target_compatible_with = [
-        "@bazel_tools//platforms:linux",
-        "@bazel_tools//platforms:x86_64",
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
     ],
     toolchain = ":host_cc_k8_toolchain",
     toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",