Revert "Move bootstrap build to MSBuild 14.0"

This reverts commit bb02897.

cibuild.cmd

@@ -109,10 +109,7 @@ if defined TestPerfRun (
     exit /b 0
 )
 
-REM Temporarily forcing MSBuild 14.0 here to work around a bug in 15.0
-REM MSBuild bug: https://github.com/Microsoft/msbuild/issues/1183
-REM Roslyn tracking bug: https://github.com/dotnet/roslyn/issues/14451
-"c:\Program Files (x86)\MSBuild\14.0\bin\MSBuild.exe" %MSBuildAdditionalCommandLineArgs% /p:BootstrapBuildPath="%bindir%\Bootstrap" BuildAndTest.proj /p:Configuration=%BuildConfiguration% /p:Test64=%Test64% /p:TestVsi=%TestVsi% /p:RunProcessWatchdog=%RunProcessWatchdog% /p:BuildStartTime=%BuildStartTime% /p:"ProcDumpExe=%ProcDumpExe%" /p:BuildTimeLimit=%BuildTimeLimit% /p:PathMap="%RoslynRoot%=q:\roslyn" /p:Feature=pdb-path-determinism /fileloggerparameters:LogFile="%bindir%\Build.log";verbosity=diagnostic /p:DeployExtension=false || goto :BuildFailed
+msbuild %MSBuildAdditionalCommandLineArgs% /p:BootstrapBuildPath="%bindir%\Bootstrap" BuildAndTest.proj /p:Configuration=%BuildConfiguration% /p:Test64=%Test64% /p:TestVsi=%TestVsi% /p:RunProcessWatchdog=%RunProcessWatchdog% /p:BuildStartTime=%BuildStartTime% /p:"ProcDumpExe=%ProcDumpExe%" /p:BuildTimeLimit=%BuildTimeLimit% /p:PathMap="%RoslynRoot%=q:\roslyn" /p:Feature=pdb-path-determinism /fileloggerparameters:LogFile="%bindir%\Build.log";verbosity=diagnostic /p:DeployExtension=false || goto :BuildFailed
 powershell -noprofile -executionPolicy RemoteSigned -file "%RoslynRoot%\build\scripts\check-msbuild.ps1" "%bindir%\Build.log" || goto :BuildFailed
 
 call :TerminateBuildProcesses