feat: allow to configure the apt upgrade method

dist-upgrade, which was the hard-coded option, is allowed to remove
packages in order to resolve dependency conflicts. This might cause
quite a mess ... it is left as the default, but can be modified by the
option apt_command.

config.example.toml

@@ -167,6 +167,8 @@
 # Extra Home Manager arguments
 # home_manager_arguments = ["--flake", "file"]
 
+# subcommand to use for apt variants != nala (dist-upgrade (default) or upgrade)
+# apt_command = "dist-upgrade"
 
 [git]
 # How many repos to pull at max in parallel

src/config.rs

@@ -313,6 +313,7 @@ pub struct Linux {
     nix_env_arguments: Option<String>,
 
     #[merge(strategy = crate::utils::merge_strategies::string_append_opt)]
+    apt_command: Option<String>,
     apt_arguments: Option<String>,
 
     enable_tlmgr: Option<bool>,
@@ -1228,6 +1229,15 @@ impl Config {
             .unwrap_or("")
     }
 
+    /// apt command: upgrade or dist-upgrade
+    pub fn apt_command(&self) -> &str {
+        self.config_file
+            .linux
+            .as_ref()
+            .and_then(|linux| linux.apt_command.as_deref())
+            .unwrap_or("dist-upgrade")
+    }
+
     /// Extra apt arguments
     pub fn apt_arguments(&self) -> Option<&str> {
         self.config_file

src/steps/os/linux.rs

@@ -518,7 +518,8 @@ fn upgrade_debian(ctx: &ExecutionContext) -> Result<()> {
     if is_nala {
         command.arg("upgrade");
     } else {
-        command.arg("dist-upgrade");
+        let apt_command = ctx.config().apt_command();
+        command.arg(apt_command);
     };
     if ctx.config().yes(Step::System) {
         command.arg("-y");