Set up proper environment variables for kernel-install

If we're not explicitly disabling kernel-install during package
manager invocations, let's set up the environment to make it do the
right thing instead.

mkosi/installer/__init__.py

@@ -47,6 +47,14 @@ def finalize_environment(cls, context: Context) -> dict[str, str]:
             context.config.bootable != ConfigFeature.disabled
         ):
             env["KERNEL_INSTALL_BYPASS"] = "1"
+        else:
+            env |= {
+                "BOOT_ROOT": "/boot",
+                # Required to make 90-loaderentry.install put the right paths into the bootloader entry.
+                "BOOT_MNT": "/boot",
+                # Hack to tell dracut to not create a hostonly initrd when it's invoked by kernel-install.
+                "hostonly_l": "no",
+            }
 
         return env