add the same non-windows msbuild check that we do for fsc and fsi

[baronfel]

One more, I somehow missed msbuild:

before(dotnet):

➜  release_netcore git:(417ecd0) ✗ dotnet fsautocomplete.dll
compilerlocation
{"Kind":"compilerlocation","Data":{"Fsc":"fsharpc","Fsi":"fsharpi","MSBuild":"MSBuild.exe"}}

before(mono):

➜  release_netcore git:(417ecd0) ✗ cd ../release
➜  release git:(417ecd0) ✗ mono fsautocomplete.exe
compilerlocation
{"Kind":"compilerlocation","Data":{"Fsc":"fsharpc","Fsi":"fsharpi","MSBuild":"msbuild"}}

after(dotnet):

➜  FsAutoComplete.netcore git:(fix-netcore-msbuild-path) dotnet run
compilerlocation
{"Kind":"compilerlocation","Data":{"Fsc":"fsharpc","Fsi":"fsharpi","MSBuild":"msbuild"}}

after(mono):

➜  net461 git:(fix-netcore-msbuild-path) mono fsautocomplete.exe
pilerlocation
{"Kind":"compilerlocation","Data":{"Fsc":"fsharpc","Fsi":"fsharpi","MSBuild":"msbuild"}}

[baronfel]

All the errors seem to be around a paket-load-script rewrite issue...