webapp: autocompile script needs to run yarn within webapp dir

we need to change the working directory to the webapp directory such
that corepack installs and uses the expected version of yarn. otherwise,
corepack installs a copy of yarn into the repository root directory.

pio-scripts/compile_webapp.py

@@ -37,22 +37,25 @@ def check_files(directories, filepaths, hash_file):
     # "exectuable" (a shell script) is only performed by cmd.exe, not by
     # Python itself. as we are calling yarn with fixed arguments, using
     # shell=True is fine.
+    # we need to change the working directory to the webapp directory such
+    # that corepack installs and uses the expected version of yarn. otherwise,
+    # corepack installs a copy of yarn into the repository root directory.
     yarn = "yarn"
     try:
-        subprocess.check_output(yarn + " --version", shell=True)
+        subprocess.check_output(yarn + " --version", cwd="webapp", shell=True)
     except FileNotFoundError:
         yarn = "yarnpkg"
         try:
-            subprocess.check_output(yarn + " --version", shell=True)
+            subprocess.check_output(yarn + " --version", cwd="webapp", shell=True)
         except FileNotFoundError:
             raise Exception("it seems neither 'yarn' nor 'yarnpkg' is available on your system")
 
     # if these commands fail, an exception will prevent us from
     # persisting the current hashes => commands will be executed again
-    subprocess.run(yarn + " --cwd webapp install --frozen-lockfile",
-                   check=True, shell=True)
+    subprocess.run(yarn + " install --frozen-lockfile",
+                   cwd="webapp", check=True, shell=True)
 
-    subprocess.run(yarn + " --cwd webapp build", check=True, shell=True)
+    subprocess.run(yarn + " build", cwd="webapp", check=True, shell=True)
 
     with open(hash_file, 'wb') as f:
         pickle.dump(file_hashes, f)