MKD_REQUIREMENTS not working whith multiple python package

Describe the bug

MKD_REQUIREMENTS variable does not work with multiple packages pipeline job will always fail

Occured in 1.4.1

Expected behavior

MKD_REQUIREMENTS is expected to be installed correctly by pip

Actual behavior

Executing pipeline job with MKD_REQUIREMENTS set with multiple values:

$ prepare_mkdocs
[INFO] installing requirements from variable 'MKD_REQUIREMENTS': mkdocs pymdown-extensions mkdocs-material pygments-graphql
ERROR: Invalid requirement: 'mkdocs pymdown-extensions mkdocs-material pygments-graphql'

Root cause

The root cause is the pip install instruction on the mkdocs template:

pip install "${MKD_REQUIREMENTS}" ${PIP_OPTS}

It is failing due to the quotes around MKD_REQUIREMENTS, reproduced on my laptop:

With Quotes:

PS C:\Users\\Documents\git\analytics\analytics> pip install "mkdocs pymdown-extensions mkdocs-material pygments-graphql"
ERROR: Invalid requirement: 'mkdocs pymdown-extensions mkdocs-material pygments-graphql'

Without quotes:

pip install mkdocs pymdown-extensions mkdocs-material pygments-graphql
Requirement already satisfied: mkdocs in c:\users\\appdata\local\programs\python\python39\lib\site-packages (1.2.2)
Requirement already satisfied: pymdown-extensions in c:\users\\appdata\local\programs\python\python39\lib\site-packages (8.2)
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