Linux and macOS using bash in a terminal window:
$ export VAMP_PATH=/path/to/plugin/directory
$ sonic-annotator -l
(or whatever sonic-annotator options you want)
Windows using Command Prompt:
set VAMP_PATH=/path/to/plugin/directory
sonic-annotator -l
Windows using PowerShell:
$env:VAMP_PATH = "/path/to/plugin/directory"
& sonic-annotator -l
Note that in all cases, setting the path to "." will make Sonic Annotator read plugins from the current working directory at the time you run it. If you have plugins + sonic-annotator binary in the same directory, you can cd to that directory and run (in bash) the one-liner
$ VAMP_PATH=. ./sonic-annotator -l