drivers: refactor pre-req install for debug mode

subiquity/server/ubuntu_drivers.py

@@ -209,6 +209,8 @@ def __init__(self, app, gpgpu: bool) -> None:
 
         self.sys_env = system_scripts_env()
 
+        self.pre_req_cmd = ["apt", "install", "-y", "umockdev", "gir1.2-umockdev-1.0"]
+
         self.list_drivers_cmd = prefix + self.list_drivers_cmd
         self.list_oem_cmd = prefix + self.list_oem_cmd
         self.install_drivers_cmd = prefix + self.install_drivers_cmd
@@ -224,9 +226,7 @@ async def ensure_cmd_exists(self, root_dir: str) -> None:
             )
         # Install wrapper script prerequisites on live system
         try:
-            await arun_command(
-                ["apt", "install", "-y", "umockdev", "gir1.2-umockdev-1.0"], check=True
-            )
+            await arun_command(self.pre_req_cmd, check=True)
         except subprocess.CalledProcessError as err:
             log.debug(f"ensure_cmd retuned with exit code {err.returncode}")
             log.debug(f"ensure_cmd stdout: {err.stdout}")
@@ -287,7 +287,7 @@ async def install_drivers(self, root_dir: str, context) -> None:
             "-t",
             root_dir,
             "--",
-            *["apt", "install", "-y", "umockdev", "gir1.2-umockdev-1.0"],
+            *self.pre_req_cmd,
             private_mounts=True,
         )