doc: Add the in-tree-build option to install docs

This is required for the setuptools_scm package to correctly identify
the root of the git repository when installing from the
requirements.txt file in the root. It'd otherwise copy the source
directory, which doesn't yet include the version metadata, into a
separate directory and the building from there, which breaks the
git lookup.

`in-tree-build` is the future default, so we'll eventually be able to
strip that option again. See [1] for details.

[1] pypa/pip#7555

doc/INSTALL.md

@@ -58,7 +58,7 @@ Clone lightning:
 For development or running tests, get additional dependencies:
 
     sudo apt-get install -y valgrind python3-pip libpq-dev
-    sudo pip3 install -r requirements.txt
+    sudo pip3 install -r requirements.txt --use-feature=in-tree-build
 
 Build lightning: