save some CI diskspace, use ubuntu 20.04 and later libgit2 (#6925)

Summary:
Pull Request resolved: #6925

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

X-link: facebook/sapling#693

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

 ---
Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/sapling/pull/693).
* __->__ facebook/sapling#693
* facebook/sapling#696
* facebook/sapling#692
* facebook/sapling#691
* facebook/sapling#682
* facebook/sapling#689
* facebook/sapling#697
* facebook/sapling#706
* facebook/sapling#730

Reviewed By: quark-zju, xavierd

Differential Revision: D49874384

Pulled By: genevievehelsel

fbshipit-source-id: 09c01eec69446c9bfa95861d9e71617b44b44fce

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