Use String instead of OsString for environment variables in config

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rustic-backup"
-version = "0.1.0"
+version = "0.1.1"
 description = "Restic wrapper for convenient backups"
 authors = ["Ben Navetta <[email protected]>"]
 readme = "README.md"

src/restic.rs

@@ -148,15 +148,15 @@ fn add_credentials(profile: &Profile, env: &mut HashMap<OsString, OsString>) ->
                 environment_file.display()
             )
         })?;
-        let env_vars: HashMap<OsString, OsString> =
+        let env_vars: HashMap<String, String> =
             toml::from_str(&env_contents).with_context(|| {
                 format!(
                     "Could not parse environment file {}",
                     environment_file.display()
                 )
             })?;
         for (var, value) in env_vars {
-            env.insert(var, value);
+            env.insert(var.into(), value.into());
         }
     }