Update bootstrap documentation to include --host_javabase

Our instruction on how to install bazel from source tell to install a
JDK on the host machine. Then the standard expectation is that this
is used, which is also the standard workflow for packaging software.
To comply with this expectation, the `--host_javabase` argument
has to be set. Fix our documentation accordingly.

Discussed on bazelbuild#6656.

Change-Id: Ib64b35d0b82fee83fcd9282519e16807588ed8a7
PiperOrigin-RevId: 230694090

site/docs/install-compile-source.md

@@ -91,7 +91,7 @@ sudo apt-get install build-essential openjdk-8-jdk python zip unzip
 
 3.  `cd` to the directory where you unpacked the distribution archive.
 
-3.  Run the compilation script: `bash ./compile.sh`.
+3.  Run the compilation script: `env EXTRA_BAZEL_ARGS="--host_javabase=@local_jdk//:jdk" bash ./compile.sh`.
 
 The compiled output is placed into `output/bazel`. This is a self-contained
 Bazel binary, without an embedded JDK. You can copy it anywhere or use it