fix: use tmp dir if $HOME isn't writable

src/alire/alire-platforms-common.adb

@@ -1,4 +1,5 @@
 with AAA.Enum_Tools;
+with GNAT.OS_Lib;
 
 with Alire.OS_Lib.Subprocess;
 
@@ -71,4 +72,27 @@ package body Alire.Platforms.Common is
       end Detect;
    end Machine_Hardware_Name;
 
+   ----------------------
+   -- Unix_Home_Folder --
+   ----------------------
+
+   function Unix_Home_Folder return String
+   is
+      Home_Var : constant String := OS_Lib.Getenv ("HOME", "unset");
+      Maybe_Windows : constant Boolean := Home_Var = "unset"
+         and then GNAT.OS_Lib.Directory_Separator = '\';
+   begin
+      if Maybe_Windows then
+         raise Checked_Error with
+            "$HOME is not set, you might be running an"
+            & " `alr` built for a non-Windows OS";
+      elsif Home_Var = "unset" or else
+       not GNAT.OS_Lib.Is_Write_Accessible_File (Home_Var)
+      then
+         return "/tmp";
+      else
+         return Home_Var;
+      end if;
+   end Unix_Home_Folder;
+
 end Alire.Platforms.Common;

src/alire/alire-platforms-common.ads

@@ -19,15 +19,7 @@ private package Alire.Platforms.Common is
    -- Unix_Home_Folder --
    ---------------------
 
-   function Unix_Home_Folder return String
-   is (if OS_Lib.Getenv ("HOME", "unset") = "unset" and then
-       GNAT.OS_Lib.Directory_Separator = '\'
-       then
-          raise Checked_Error with
-            "$HOME is not set, you might be running an"
-       & " `alr` built for a non-Windows OS"
-       else
-          OS_Lib.Getenv ("HOME", Default => "/tmp"));
+   function Unix_Home_Folder return String;
 
    ----------------------
    -- Unix_Temp_Folder --