Use latest build-su2-cross Docker image for release-management Github Actions workflow

[martin-g]

Proposed Changes

build-su2-cross should extend from build-su2 Docker image to use the same GCC version.

Failed build: https://github.com/su2code/SU2/runs/7125551739?check_suite_focus=true

The PR is draft because it depends on the build of su2code/Docker-Builds#7

Related Work

#1619 - Updates Ubuntu to 20.04 and GCC from 7.x to 9.x.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.