Keep output machine readable using spack find --format in an env (#19698)

Currently, full JSON output is the only machine readable option for `spack find`
in an environment.

`spack find --format` is also designed to be machine readable, but we print extra
headers in environments.

-[x] don't print headers in `spack find` output when in an environment

lib/spack/spack/cmd/find.py

@@ -235,6 +235,7 @@ def find(parser, args):
     if args.json:
         cmd.display_specs_as_json(results, deps=args.deps)
     else:
+        if not args.format:
             if env:
                 display_env(env, args, decorator)
         if sys.stdout.isatty() and args.groups:

lib/spack/spack/test/cmd/find.py

@@ -209,6 +209,7 @@ def test_find_format(database, config):
 
     output = find('--format', '{name}-{version}-{compiler.name}-{^mpi.name}',
                   'mpileaks')
+    assert "installed package" not in output
     assert set(output.strip().split('\n')) == set([
         "mpileaks-2.3-gcc-zmpi",
         "mpileaks-2.3-gcc-mpich",