Fix "Lint Code Base" shellcheck workflow errors

[ScottBrenner]

Fix the failing checks from the "Lint Code Base" shellcheck workflow step:

File:[./entrypoint.sh]
ERROR! Found errors in [shellcheck] linter!
ERROR:[
In ./entrypoint.sh line 3:
git clone --quiet https://github.com/$REPO &> /dev/null
                                     ^---^ SC2086: Double quote to prevent globbing and word splitting.
                                           ^----------^ SC2039: In POSIX sh, &> is undefined.

Did you mean: 
git clone --quiet https://github.com/"$REPO" &> /dev/null


In ./entrypoint.sh line 12:
echo $changelog
     ^--------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
echo "$changelog"


In ./entrypoint.sh line 14:
changelog="${changelog//'%'/'%25'}"
           ^---------------------^ SC2039: In POSIX sh, string replacement is undefined.


In ./entrypoint.sh line 15:
changelog="${changelog//$'\n'/'%0A'}"
           ^-----------------------^ SC2039: In POSIX sh, string replacement is undefined.
                        ^---^ SC2039: In POSIX sh, $'..' is undefined.


In ./entrypoint.sh line 16:
changelog="${changelog//$'\r'/'%0D'}"
           ^-----------------------^ SC2039: In POSIX sh, string replacement is undefined.
                        ^---^ SC2039: In POSIX sh, $'..' is undefined.

For more information:
  https://www.shellcheck.net/wiki/SC2039 -- In POSIX sh, $'..' is undefined.
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...]

https://github.com/ScottBrenner/generate-changelog-action/runs/1129086107?check_suite_focus=true#step:4:71