[dirmd] Fix incorrect DIR_METADATA

The correct OS is "LINUX" not "OS_LINUX"
Proto: https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/tools/dirmd/proto/dir_metadata.proto;l=49?q=dir_metadata.proto

Bug: 1106176
Change-Id: If66b4e4c0f12315c428ebb2c73268b6e92e1955a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303467
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789465}

tools/binary_size/libsupersize/testdata/mock_source_directory/base/DIR_METADATA

@@ -3,4 +3,4 @@ monorail {
   component: "Blink>Internal"
 }
 team_email: "team@chromium.org"
-os: OS_LINUX
+os: LINUX