Packing fails when repo path has spaces in it

```
Packaging ...
MSBUILD : error MSB1008: Only one project can be specified. [C:\Users\micodoba\.nuget\packages\roslyntools.repotoolset\1.0.0-beta-62512-02\tools\Build.proj]
Switch: spaces\msbuild\

For switch syntax, type "MSBuild /help"
C:\Users\micodoba\.nuget\packages\roslyntools.repotoolset\1.0.0-beta-62512-02\tools\Build.proj(191,5): error MSB3073: The command ""C:\Program Files (x86)\Microsoft Visual Studio\Preview\Enterprise\MSBuild\15.0\bin\MSBuild.exe" /nodeRe
use:false "E:\projects\with spaces\msbuild\MSBuild.sln" /nologo /m /v:minimal /t:Pack /p:NoBuild=true /bl:"E:\projects\with spaces\msbuild\artifacts\Debug\log\Pack.binlog" /p:Configuration=Debug /p:CIBuild=True /p:RepoRoot=E:\projects\
with spaces\msbuild\ /p:VersionsPropsPath=E:\projects\with spaces\msbuild\build\Versions.props /p:DotNetPackageVersionPropsPath= /p:__BuildPhase=SolutionBuild" exited with code 1.
```

Root path is `e:\projects\with spaces\msbuild`

Arguments to the failing Exec task

```
"C:\Program Files (x86)\Microsoft Visual Studio\Preview\Enterprise\MSBuild\15.0\bin\MSBuild.exe" /nodeReuse:false "E:\projects\with spaces\msbuild\MSBuild.sln" /nologo /m /v:minimal /t:Pack /p:NoBuild=true /bl:"E:\projects\with spaces\msbuild\artifacts\Debug\log\Pack.binlog" /p:Configuration=Debug /p:CIBuild=True /p:RepoRoot=E:\projects\with spaces\msbuild\ /p:VersionsPropsPath=E:\projects\with spaces\msbuild\build\Versions.props /p:DotNetPackageVersionPropsPath= /p:__BuildPhase=SolutionBuild
```