.github/workflows/main.yml: run static-analysis on bionic

.github/workflows/main.yml: run static-analysis on bionic

GitHub Actions is transitioning workflow steps that run on
'ubuntu-latest' from 18.04 to 20.04 [1].

This works fine in all steps except the static-analysis one, since
Coccinelle isn't available on Ubuntu focal (it is only available in the
universe suite).

Until Coccinelle can be installed from 20.04's main suite, pin the
static-analysis build to run on 18.04, where it can be installed by
default.

[1]: https://github.com/actions/virtual-environments/issues/1816

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>