[release/2.1] Fixup NPM package versioning (#30164)

- `_Pack` target failing in ProdCon builds
  - fortunately problem is specific to non-`IsFinalBuild` builds
- don't add the version metadata because that's not written into package.lock
- e.g. looking for aspnet-signalr-protocol-msgpack-1.0.26-servicing-20210212.12+pb-20210212-02.tgz
  - but `npm pack` actually wrote aspnet-signalr-protocol-msgpack-1.0.26-servicing-20210212.12.tgz
- see https://dnceng.visualstudio.com/internal/_build/results?buildId=992921&view=logs&j=f31c9f97-4411-58e7-49ac-fc73f645e6b6&t=2bcaa12b-2f4b-5b1f-c519-10308f653190&l=3190

Author: dougbu

version.props

@@ -39,7 +39,9 @@
     <ExperimentalPackageVersion  Condition=" '$(IncludePreReleaseLabelInPackageVersion)' == 'true' ">$(ExperimentalVersionPrefix)-$(VersionSuffix)</ExperimentalPackageVersion>
 
     <VersionMetadata Condition=" '$(DotNetProductBuildId)' != '' ">pb-$(DotNetProductBuildId)</VersionMetadata>
-    <VersionSuffix Condition=" '$(VersionMetadata)' != '' ">$(VersionSuffix)+$(VersionMetadata)</VersionSuffix>
+
+    <!-- `npm` will truncate the '+' and everything after when updating project.json. Condition avoids _Pack failures. -->
+    <VersionSuffix Condition=" '$(VersionMetadata)' != '' AND '$(MSBuildProjectExtension)' != '.npmproj' ">$(VersionSuffix)+$(VersionMetadata)</VersionSuffix>
 
     <SharedFxCliBlobChannel>release/$(AspNetCoreMajorVersion).$(AspNetCoreMinorVersion)</SharedFxCliBlobChannel>