Lowercase org name when constructing docker tag names

[joeyparrish]

This resolves errors like:

Error: buildx failed with: ERROR: invalid tag "ghcr.io/Stephane-D/sgdk-m68k-gcc:latest": repository name must be lowercase

For example, see https://github.com/Stephane-D/SGDK/actions/runs/8545289521/job/23413232803

This also fixes several additional workflow issues that occurred during debugging and development:

• Build logs from crosstool-ng were overwhelming (180k+ useless lines of a progress bar), now disabled with CT_LOG_PROGRESS_BAR=n
• Bootstrapping for first GCC image on main repo or fork was broken, now fixed to ensure workflow's GCC image used in later SGDK image.