fix(ci): restore [toolchain] moc = "1.5.0" in backend/mo/ic_vetkeys/mops.toml

The toolchain section was added in the base branch (2c7813b) to allow
mops test to resolve the Motoko compiler without falling back to
`dfx cache show` (dfx is not installed). Our patch overwrote the file
with the version from all-changes which was missing this section,
causing backend-motoko CI jobs to fail with "dfx: not found".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marc0olo

backend/mo/ic_vetkeys/mops.toml

@@ -1,3 +1,6 @@
+[toolchain]
+moc = "1.5.0"
+
 [package]
 name = "ic-vetkeys"
 version = "0.4.0"