Add support to change lint output format from environment

Use REUSE_OUTPUT_FORMAT to allow overriding lint output formats in certain
environments (like CI).

changelog.d/added/format-env.md

@@ -0,0 +1,2 @@
+- Added `REUSE_OUTPUT_FORMAT` environment variable to configure output for
+  `lint`.

docs/man/reuse-lint.rst

@@ -103,3 +103,12 @@ Options
 .. option:: -h, --help
 
   Display help and exit.
+
+Environment
+-----------
+
+.. envvar:: REUSE_OUTPUT_FORMAT
+
+   Specifies output format, one of ``plain``, ``lines``, ``github``, ``json``
+
+   It behaves same as corresponding command line options.

src/reuse/lint.py

@@ -13,6 +13,7 @@
 from __future__ import annotations
 
 import json
+import os
 import sys
 from argparse import ArgumentParser, Namespace
 from gettext import gettext as _
@@ -376,15 +377,26 @@ def run(args: Namespace, project: Project, out: IO[str] = sys.stdout) -> int:
         project, do_checksum=False, multiprocessing=not args.no_multiprocessing
     )
 
-    if args.quiet:
-        pass
-    elif args.json:
-        out.write(format_json(report))
-    elif args.lines:
-        out.write(format_lines(report))
-    elif args.github:
-        out.write(format_github(report))
-    else:
-        out.write(format_plain(report))
+    formatters = {
+        "json": format_json,
+        "lines": format_lines,
+        "github": format_github,
+        "plain": format_plain,
+    }
+
+    if not args.quiet:
+        output_format = os.environ.get("REUSE_OUTPUT_FORMAT")
+
+        if output_format is not None and output_format in formatters:
+            formatter = formatters[output_format]
+            out.write(formatter(report))
+        elif args.json:
+            out.write(format_json(report))
+        elif args.lines:
+            out.write(format_lines(report))
+        elif args.github:
+            out.write(format_github(report))
+        else:
+            out.write(format_plain(report))
 
     return 0 if report.is_compliant else 1