Jenkins: How to modify PATH environment variable for build steps?

Jenkins: How to modify PATH environment variable for build steps?

I'm trying to follow the instructions here to prepend a directory to path
for my build steps. However, the instructions reference the deprecated
SetEnv plugin. I tried playing with the new EnvInject plugin, setting
PATH=mydir:$PATH in Script Content field of the "Inject Build Environments
to the build process" section. However, the path is not updated when my
build step shell scripts execute.