Partially revert 0e56e4f

Rollback the build environment from Ubuntu 22.04 to 18.04, except for riscv64 that isn't supported by Ubuntu 18.04.

Fix issue 7255 (`1.6.7 can't be run on Ubuntu LTS 20.04 (GLIBC_2.34 not found)`)

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 4818610)
Signed-off-by: Samuel Karp <samuelkarp@google.com>

[release/1.6] release: rollback Ubuntu to 18.04 (except for riscv64)

In the 1.6.7 release, we saw significantly longer execution time for
producing builds that exceeded the previous timeout of 10 minutes,
causing the workflow to fail.  After increasing to 20 minutes in the
release/1.6 branch, we continued to see one failure (which succeeded on
retry).

Increase to 30 minutes to provide additional buffer for the build to
complete.

Signed-off-by: Samuel Karp <samuelkarp@google.com>
(cherry picked from commit f8add92)
Signed-off-by: Samuel Karp <samuelkarp@google.com>

Release builds are performed from within a Dockerfile-defined
environment and do not require Go to be installed in the GitHub Actions
runner environment.

Signed-off-by: Samuel Karp <samuelkarp@google.com>
(cherry picked from commit 6ba91de)
Signed-off-by: Samuel Karp <samuelkarp@google.com>

[release/1.6] release workflow: increase timeout to 30 minutes & remove Go setup action

Signed-off-by: Derek McGowan <derek@mcg.dev>

[release/1.6] Prepare release notes for v1.6.8