qt-cpp: Add rest of the environment path to path variable

When ninja doesn't exist in PATH and on a unix based system, Ninja might
be used from the Qt installation. In this case, the following output is
generated.

Output:
`"PATH": "/home/orkun/Qt/Tools/Ninja"`

The problem is that the above variable overrides the PATH variable
completely and it causes compilers not to be found. This commit adds the
rest of the environment path to the PATH variable for unix based systems
when Ninja is not found in PATH.

qt-cpp/src/kit-manager.ts

@@ -510,7 +510,7 @@ export class KitManager {
 
   private static generateEnvPathForQtInstallation(installation: string) {
     if (!IsWindows) {
-      return undefined;
+      return envPath;
     }
     const installationBinDir = path.join(installation, 'bin');
     const QtPathAddition = [installationBinDir, envPath].join(path.delimiter);
@@ -544,7 +544,8 @@ export class KitManager {
     newKit.name = kitName;
     newKit.environmentVariables = {
       VSCODE_QT_INSTALLATION: installation,
-      PATH: qtPathEnv
+      // If it is just envPath, not need to set it.
+      PATH: qtPathEnv === envPath ? undefined : qtPathEnv
     };
 
     const toolchainFilePath = await promiseCmakeQtToolchainPath;