Fix file casing issues in .nuspecs

On *NIX we have a case sensitive file system, so we need the correct
casing in the .nuspecs.

src/NuGet/Microsoft.Net.CSharp.Interactive.netcore.nuspec

@@ -27,6 +27,6 @@
     </dependencies>
   </metadata>
   <files>
-    <file src="Exes/csicore/csi.exe" target="runtimes/any/native" />
+    <file src="Exes/CsiCore/csi.exe" target="runtimes/any/native" />
   </files>
 </package>

src/NuGet/Microsoft.Net.Compilers.netcore.nuspec

@@ -29,7 +29,7 @@
   </metadata>
   <files>
     <file src="$emptyDirPath$/_._" target="ref/netcoreapp1.0" />
-    <file src="Exes/csccore/csc.exe" target="runtimes/any/native" />
-    <file src="Exes/vbccore/vbc.exe" target="runtimes/any/native" />
+    <file src="Exes/CscCore/csc.exe" target="runtimes/any/native" />
+    <file src="Exes/VbcCore/vbc.exe" target="runtimes/any/native" />
   </files>
 </package>