textproc/py-ocrmypdf: Use defined locale in whole build process

• Set the locale globally for {CONFIGURE,MAKE}_ENV so that the locale doesn't need to be set explicitly for the do-test target.

• Use ECHO_CMD instead of ECHO as the latter one is already defined in /usr/share/mk/sys.mk, which can either be "echo" or "true" if the make flag -s is given.

Suggested by: sunpoet