Enforce E501: Limit the line length to 79 for files in pmb/build
Made changes to limit the line length in following files for #1986 (closed),
- pmb/build/_package.py
- pmb/build/envkernel.py
- pmb/build/init.py
- pmb/build/other.py
Added the above files in E501 flake8 command list. Substitute f-string for string concatenation.