feat: add --set-env flag to set environment variables from command line

aider/args.py

@@ -772,6 +772,13 @@ def get_parser(default_config_files, git_root):
         "--editor",
         help="Specify which editor to use for the /editor command",
     )
+    group.add_argument(
+        "--set-env",
+        action="append",
+        metavar="ENV_VAR_NAME=value",
+        help="Set an environment variable (can be used multiple times)",
+        default=[],
+    )
 
     ##########
     group = parser.add_argument_group("Voice Settings")

aider/main.py

@@ -454,6 +454,17 @@ def main(argv=None, input=None, output=None, force_git_root=None, return_coder=F
     # Load the .env file specified in the arguments
     loaded_dotenvs = load_dotenv_files(git_root, args.env_file, args.encoding)
 
+    # Process any environment variables set via --set-env
+    if args.set_env:
+        for env_setting in args.set_env:
+            try:
+                name, value = env_setting.split('=', 1)
+                os.environ[name.strip()] = value.strip()
+            except ValueError:
+                io.tool_error(f"Invalid --set-env format: {env_setting}")
+                io.tool_output("Format should be: ENV_VAR_NAME=value")
+                return 1
+
     # Parse again to include any arguments that might have been defined in .env
     args = parser.parse_args(argv)