Remove explicit framework version dependency in signing

src/Compilers/Core/Desktop/Interop/ClrStrongName.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Runtime.InteropServices;
+using System.Security;
 
 namespace Microsoft.CodeAnalysis.Interop
 {
@@ -15,8 +16,6 @@ internal static class ClrStrongName
 
         internal static IClrStrongName GetInstance()
         {
-            const string fxVersion = "v4.0.30319";
-
             var metaHostClsid = new Guid(0x9280188D, 0xE8E, 0x4867, 0xB3, 0xC, 0x7F, 0xA8, 0x38, 0x84, 0xE8, 0xDE);
             var metaHostGuid = new Guid(0xD332DB9E, 0xB9B3, 0x4125, 0x82, 0x07, 0xA1, 0x48, 0x84, 0xF5, 0x32, 0x16);
             var clrStrongNameClsid = new Guid(0xB79B0ACD, 0xF5CD, 0x409b, 0xB5, 0xA5, 0xA1, 0x62, 0x44, 0x61, 0x0B, 0x92);
@@ -24,7 +23,7 @@ internal static IClrStrongName GetInstance()
             var clrStrongNameGuid = new Guid(0x9FD93CCF, 0x3280, 0x4391, 0xB3, 0xA9, 0x96, 0xE1, 0xCD, 0xE7, 0x7C, 0x8D);
 
             var metaHost = (IClrMetaHost)nCreateInterface(metaHostClsid, metaHostGuid);
-            var runtime = (IClrRuntimeInfo)metaHost.GetRuntime(fxVersion, clrRuntimeInfoGuid);
+            var runtime = (IClrRuntimeInfo)metaHost.GetRuntime(RuntimeEnvironment.GetSystemVersion(), clrRuntimeInfoGuid);
             return (IClrStrongName)runtime.GetInterface(clrStrongNameClsid, clrStrongNameGuid);
         }
     }