[CI][Dev] Fix shellcheck errors in the ci/scripts/r_test.sh

[hiroyuki-sato]

Describe the enhancement requested

This is the sub issue #44748.

• SC1091: Not following: (error message here)
• SC2086: Double quote to prevent globbing and word splitting.
• SC2223: This default assignment may cause DoS due to globbing. Quote it.

shellcheck ci/scripts/r_test.sh

In ci/scripts/r_test.sh line 21:
: ${R_BIN:=R}
  ^---------^ SC2223 (info): This default assignment may cause DoS due to globbing. Quote it.


In ci/scripts/r_test.sh line 25:
pushd ${source_dir}
      ^-----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean:
pushd "${source_dir}"


In ci/scripts/r_test.sh line 30:
  . "${ARROW_PYTHON_VENV}/bin/activate"
    ^-- SC1091 (info): Not following: ./bin/activate: openBinaryFile: does not exist (No such file or directory)

For more information:
  https://www.shellcheck.net/wiki/SC1091 -- Not following: ./bin/activate: op...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2223 -- This default assignment may cause...

Component(s)

Continuous Integration

[KHARSHAVARDHAN-eng]

Hi maintainers, I’ve raised this PR for review. Whenever you have the time, please take a look. If there are any issues, improvements, or changes you’d like me to make, I’d be happy to address them. I’ll be waiting for your feedback. Thank you!

[kou]

Issue resolved by pull request 50795
#50795