source: fix Git extraction path for CMake recipes

When trying to build a recipe that uses both git and CMake, the following error message is shown:
CMake Error: The source directory "/home/jorge/cerbero/sources/android_universal/armv7/my-package-1.2" does not appear to contain CMakeLists.txt.

When using CMake:
self.build_dir == ARCH/PACKAGE-VERSION/_builddir (Path where Git extracts sources)
and
self.config_src_dir == ARCH/PACKAGE-VERSION (Path where Git should extract sources)

The problem is that Git class "extracts" sources to self.build_dir.
build_dir is the path to put binaries (as explained in the message of commit 2d520bfd)

Sources have to be extracted to self.config.sources:
- Tarball Class extracts the sources to self.config.sources.
- self.config.sources is also the path where patches are applied.

Part-of: <!700>
20 jobs for master in 100 minutes and 42 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #9651869

00:00:11

passed android docker #9651868

00:00:14

passed cerbero fedora amd64 docker #9651867

00:00:15

passed fedora amd64 docker #9651866

00:00:16

passed gst-indent amd64 docker #9651870

00:00:15

passed windows amd64 docker #9651871
windows shell 1809

00:00:09

 
  Preparation
passed manifest #9651872

00:00:11

 
  Build
passed cerbero deps cross-android universal #9651874
gstreamer

00:30:12

passed cerbero deps cross-ios universal #9651878
gst-ios-14.3

00:51:44

passed cerbero deps cross-windows x86 #9651875
gstreamer

00:11:51

passed cerbero deps cross-windows x86_64 #9651876
gstreamer

00:13:35

passed cerbero deps fedora x86_64 #9651873
gstreamer

00:10:25

passed cerbero deps macos x86_64 #9651877
gst-macos-11.1

00:34:11

passed cerbero deps msvc x86_64 #9684195
docker windows 1809 gstreamer-windows

00:45:11

failed cerbero deps msvc x86_64 #9651879
docker windows gstreamer-windows 1809

02:25:40

failed cerbero deps msvc x86_64 #9656961
docker windows 1809 gstreamer-windows

00:02:11

failed cerbero deps msvc x86_64 #9683703
docker windows 1809 gstreamer-windows

00:01:45

failed cerbero deps msvc x86_64 #9683519
docker windows 1809 gstreamer-windows

00:01:56

 
  Integrate
passed cerbero cross-android universal examples #9651880

00:06:31

passed cerbero cross-ios universal examples #9651881
gst-ios-14.3

00:03:19