sideload: get channel from package not from global pmaports (MR 2569)

Currently packages that are not in the systemd channel cannot be
sideloaded when using the systemd channel. This is because apk sideload
uses the global aports channel and not the channel of the package. Fix
it with this patch.

pmb/sideload/__init__.py

@@ -108,7 +108,6 @@ def sideload(
     :param pkgnames: list of pkgnames to be built"""
 
     paths = []
-    channel: str = pmb.config.pmaports.read_config()["channel"]
 
     if arch is None:
         arch = ssh_find_arch(args, user, host, port)
@@ -121,8 +120,12 @@ def sideload(
         if data_repo is None:
             raise RuntimeError(f"Couldn't find APKINDEX data for {pkgname}!")
 
+        base_aports, _ = pmb.build.get_apkbuild(pkgname)
+        channel = pmb.config.pmaports.read_config(base_aports)["channel"]
+
         apk_file = f"{pkgname}-{data_repo.version}.apk"
         host_path = context.config.work / "packages" / channel / arch / apk_file
+
         if not host_path.is_file():
             to_build.append(pkgname)