Don't bump MSBuild dependency

Since we want to support running on VS2017+, we can't bump to newer MSBuild dep.

.github/dependabot.yml

@@ -6,4 +6,6 @@ updates:
 - package-ecosystem: nuget
   directory: /
   schedule:
-    interval: weekly
+    interval: weekly
+  ignore:
+    - dependency-name: "Microsoft.Build.Utilities.Core"

.netconfig

@@ -44,9 +44,7 @@
 	weak
 [file ".github/dependabot.yml"]
 	url = https://github.com/devlooped/oss/blob/main/.github/dependabot.yml
-	sha = 0683ee777d7d878d4bf013d7deea352685135a05
-	etag = 2fc8a0d2b47091b058ae3e1f68333492044b49a684621f4939a0bce5bff869d5
-	weak
+    skip
 [file ".github/workflows/build.yml"]
 	url = https://github.com/devlooped/oss/blob/main/.github/workflows/build.yml
 	sha = fc5889d5387e2d5aa7aba279b2aa12251cf08cb2