Add Ubuntu 18.04 image for testing older GCC #991

Job	Run time
clippy (aws-lc-rs, --features bindgen,unstable)	1m 31s
dependency-review	27s
clippy (aws-lc-rs, --features bindgen,unstable,fips)	1m 51s
bindgen-dependency (ubuntu-latest)	31s
apidiff (aws-lc-sys, branch)	2m 35s
clippy (aws-lc-rs, --no-default-features --features aws-lc-sys)	1m 2s
bindgen-dependency (macos-12)	46s
apidiff (aws-lc-sys, published)	2m 39s
mirai-analysis	2m 32s
bindgen-dependency (macos-13-xlarge)	35s
apidiff (aws-lc-fips-sys, branch)	2m 42s
rustfmt	36s
bindgen-dependency (windows-latest)	1m 5s
apidiff (aws-lc-fips-sys, published)	3m 6s
udeps	7m 34s
apidiff (aws-lc-rs, branch)	2m 29s
apidiff (aws-lc-rs, published)	2m 44s
bindgen-fips-dependency (ubuntu-latest)	32s
copyright	6s
bindgen-fips-dependency (macos-12)	39s
bindgen-fips-dependency (macos-13-xlarge)	39s
metadata-checks	39s
bindgen-fips-dependency (windows-latest)	1m 24s
Resolve the dependencies to the minimum SemVer version (aws-lc-rs)	54s
semver-checks	1m 29s
Resolve the dependencies to the minimum SemVer version (aws-lc-sys)	51s
Resolve the dependencies to the minimum SemVer version (aws-lc-fips-sys)	1m 16s
	43m 14s

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