App.os_cascade: Don't use platform.system()

Python's platform.system() outputs 'Linux', and to use it is
tempting. Sadly, that's wrong, because it reflects the host's idea of
the target system, not the execution context's, so replace it with
straight 'linux' if the distro is known, or, failing that, the output
of uname -s.

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/App.py

@@ -300,15 +300,31 @@ class App(Base):
     @property
     def os_release(self) -> str:
         if self.__os_release is None:
-            result = self.call_async(
+            release_file = '/etc/os-release'
-                        self.exec_context.run(
+            try:
-                            ['/usr/bin/cat', '/etc/os-release'],
+                result = self.call_async(
-                            throw=True,
+                            self.exec_context.run(
-                            cmd_input=InputMode.NonInteractive
+                                ['/usr/bin/cat', release_file],
+                                throw=True,
+                                cmd_input=InputMode.NonInteractive
+                            )
                         )
-                    )
+                self.__os_release = result.decode().stdout.strip()
-            assert result.status == 0
+            except Exception as e:
-            self.__os_release = result.decode().stdout.strip()
+                result = self.call_async(
+                            self.exec_context.run(
+                                ['uname', '-s'],
+                                throw=False,
+                                cmd_input=InputMode.NonInteractive
+                            )
+                        )
+                if result.status != 0:
+                    log(ERR, f'/etc/os-release and uname both failed, the latter with exit status {result.status}')
+                    raise
+                log(INFO, f'Failed to read {release_file} ({str(e)}), falling back to uname')
+                uname = result.decode().stdout.strip().lower()
+                self.__os_release = f'ID={uname}\nVERSION_CODENAME=unknown'
+
         return self.__os_release
 
     def os_release_field(self, key: str, throw: bool=False) -> str:
@@ -422,32 +438,38 @@ class App(Base):
         def __append(entry: str):
             if not entry in ret:
                 ret.append(entry)
-        import platform
         if self.__os_cascade is None:
-            ret = [ 'os', platform.system().lower() ]
+            ret = [ 'os' ]
             match self.distro_id:
                 case 'centos':
+                    __append('linux')
                     __append('pkg-rpm')
                     __append('pm-yum')
                     __append('redhat')
                     __append('rhel')
                 case 'fedora' | 'rhel':
+                    __append('linux')
                     __append('pkg-rpm')
                     __append('pm-yum')
                     __append('redhat')
                 case 'suse':
+                    __append('linux')
                     __append('pkg-rpm')
                     __append('pm-zypper')
                 case 'kali' | 'raspbian':
+                    __append('linux')
                     __append('pkg-debian')
                     __append('pm-apt')
                     __append('debian')
                 case 'ubuntu':
+                    __append('linux')
                     __append('pkg-debian')
                     __append('pm-apt')
                 case 'archlinux':
+                    __append('linux')
                     __append('pkg-pm')
                     __append('pm-pacman')
+
             os = self.__opt_os if self.__opt_os is not None else self.get_os()
             name = re.sub(r'-.*', '', os)
             series = os