[docs] A reminder on linux doc to use $HOME on PATH

Every now and then non-googlers end up having issues  when running gclient
run hooks because they use ~ to refer to the home directory. This update
mentions that they have to use ${HOME}.

Bug: 811748
Change-Id: I5228698d363fe030ff3825371216eca40fc27ce8
Reviewed-on: https://chromium-review.googlesource.com/916283
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536368}

docs/linux_build_instructions.md

@@ -36,6 +36,14 @@ in your `~/.bashrc` or `~/.zshrc`). Assuming you cloned `depot_tools` to
 $ export PATH="$PATH:/path/to/depot_tools"
 ```
 
+When cloning `depot_tools` to your home directory **do not** use `~` on PATH,
+otherwise `gclient runhooks` will fail to run. Rather, you should use either
+`$HOME` or the absolute path:
+
+```shell
+$ export PATH="$PATH:${HOME}/depot_tools"
+```
+
 ## Get the code
 
 Create a `chromium` directory for the checkout and change to it (you can call