Simplify generation of YAML block comment in PDF

Since the OTS_DOCTOOLS_DIR environment variable is required now, use
that instead of Python's '__file__' variable to get values for the
YAML block comment that we put at the end of each PDF.

Note that the two other uses of '__file__' in this code base, in
'pipeline/pipeline.py' and 'pipeline/plugins/65_bugs.py', stay as they
are: for what they're doing, '__file__' is appropriate.