Property value is incorrectly cached

When building multiple times a project in VS15.8.2, changing the content of imported msbuild project files does not consistently work across all Target Frameworks. All TF use properly updated values, exception `xamarinios10`.

## Repro steps

- Clone https://github.com/nventive/Uno
- Rename `src/crosstargeting_override.props.sample` to `src/crosstargeting_override.props`
- Uncomment and change `UnoNugetOverrideVersion` to `1.31.0-dev.191`
- Comment the property `BuildTargetFramework`
- Open this solution: `https://github.com/nventive/Uno/blob/master/src/Uno.UI.sln`
- Re-build the project Uno.UI in this solution
- Notice the messages `“OVERRIDING NUGET PACKAGE CACHE: C:\Users\user\.nuget\packages\Uno.UI\1.31.0-dev.191\lib\MonoAndroid80”`
- In `src/crosstargeting_override.props`, change `UnoNugetOverrideVersion` to `1.31.0-dev.192`
- Rebuild Uno.UI
- Notice that part of the `OVERRIDING NUGET PACKAGE CACHE messages are using 1.31.0-dev.191` and others `1.31.0-dev.192`, inconsistently.
- The override target that print this message is here: https://github.com/nventive/Uno/blob/bf3a302a2760f328cdf69686840c637203940b0b/src/Uno.CrossTargetting.props#L90

## Workaround
Restarting VS works around the issue.