perf(ci): pin staged restore mtimes so the restore layer caches, drop build intermediates from the layer [CI]

packaging/container/Agent.NativeBuild.Containerfile

@@ -29,7 +29,8 @@ RUN dotnet publish agent/src/MoShiro.Agent -m:3 -c Release -r "$NODE_RID" \
     --self-contained true --no-restore -p:PublishAot=true -p:UseAotAnywhere=true \
     -p:StaticMsQuic=true -p:StaticExecutable=true -p:RuntimeFrameworkVersion=10.0.12 -p:InvariantGlobalization=true \
     -p:UseAppHost=true -p:StripSymbols=true \
-    -p:DebugType=none -p:DebugSymbols=false --artifacts-path /artifacts -o /out/enroll
+    -p:DebugType=none -p:DebugSymbols=false --artifacts-path /artifacts -o /out/enroll \
+    && rm -rf /artifacts
 RUN test -x /out/agent/moshiro-agent && test -x /out/enroll/fleet-enroll \
     && test "$(stat -c%s /out/agent/moshiro-agent)" -gt 2000000 \
     && test "$(stat -c%s /out/enroll/fleet-enroll)" -gt 2000000 \