When relying on the BOWTIE2_INDEXES environment variables, the check for
large bowtie index files in the current working directory introduced in
496d137be3 would affect the subsequent lookup in the directory
pointed to by BOWTIE2_INDEXES.
Also fix how paths are built (use os.path.join, to ensure the dir
separator is added if not present in the env var)
When relying on the BOWTIE2_INDEXES environment variables, the check for large bowtie index files in the current working directory introduced in 496d137be3 would affect the subsequent lookup in the directory pointed to by BOWTIE2_INDEXES.
Also fix how paths are built (use os.path.join, to ensure the dir separator is added if not present in the env var)