Before this PR, GitHub Actions workflows use GCC compilers which have been pre-installed into the self-hosted runner.
Since version management would be harder as development evolves, however, I've decided to change the management strategy to the one in which the workflows use the official docker images for GCC.
Similar workflows which use Clang compilers still uses the ones pre-installed into the self-hosted runner, which will also be changed to using some docker images.
Before this PR, GitHub Actions workflows use GCC compilers which have been pre-installed into the self-hosted runner.
Since version management would be harder as development evolves, however, I've decided to change the management strategy to the one in which the workflows use the official docker images for GCC.
Similar workflows which use Clang compilers still uses the ones pre-installed into the self-hosted runner, which will also be changed to using some docker images.