filters: append well known dirs to exec PATH

To allow referencing built-in filters without hard coding a path during
installation, append the following folders to the exec PATH when running
the filter commands:

	~/.config/aerc/filters
	~/.local/share/aerc/filters
	$PREFIX/share/aerc/filters
	/usr/share/aerc/filters

If a filter script has the same name than a command in the default exec
PATH, it will not shadow it. In that edge case, the absolute path to the
filter script must be specified.

Suggested-by: Teo Luppi <me@luppi.uk>
Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

widgets/msgviewer.go

@@ -577,6 +577,12 @@ func NewPartViewer(acct *AccountView, conf *config.AercConfig,
 		}
 	}
 	if filter != nil {
+		path, _ := os.LookupEnv("PATH")
+		for _, dir := range config.SearchDirs {
+			path += fmt.Sprintf(":%s/filters", dir)
+		}
+		filter.Env = os.Environ()
+		filter.Env = append(filter.Env, fmt.Sprintf("PATH=%s", path))
 		if pipe, err = filter.StdinPipe(); err != nil {
 			return nil, err
 		}