save some CI diskspace, use ubuntu 20.04 and later libgit2

Save some time and diskspace in CI builds for github by using the system libraries

Test plan:

Install deps and build locally with

```
sudo ./build/fbcode_builder/getdeps.py --allow-system-packages install-system-deps --recursive eden
...
./build/fbcode_builder/getdeps.py --allow-system-packages build --src-dir=. eden

```

before: we build libgit2 from source
after: we use the system library

build/fbcode_builder/manifests/libgit2

@@ -10,8 +10,8 @@ libgit2-devel
 # Ubuntu 18.04 libgit2 has clash with libcurl4-openssl-dev as it depends on
 # libcurl4-gnutls-dev.  Should be ok from 20.04 again
 # There is a description at https://github.com/r-hub/sysreqsdb/issues/77
-# [debs]
-# libgit2-dev
+[debs.not(all(distro=ubuntu,distro_vers="18.04"))]
+libgit2-dev
 
 [download]
 url = https://github.com/libgit2/libgit2/archive/v0.28.1.tar.gz