https://cs.chromium.org/chromium/src/chrome/installer/linux/debian/build.sh

# TODO(mmoss) This currently only works with official builds, since non-official
# builds don't add the "${BUILDDIR}/installer/" files needed for packaging.

The first problem is a minor bug in the script. (I guess the variable is set on build-bots.)

$ ./build.sh -h
usage: build.sh [-c channel] [-a target_arch] [-o 'dir'] 
                      [-b 'dir'] -d branding
-c channel the package channel (trunk, asan, unstable, beta, stable)
-a arch    package architecture (ia32 or x64)
-o dir     package output directory [/home/user/chromium/src/chrome/installer/linux/debian]
./build.sh: line 129: BUILDDIR: unbound variable
Cleaning...

$ export BUILDDIR=~/chromium/src/out/linux-release/

$ ./build.sh -h
usage: build.sh [-c channel] [-a target_arch] [-o 'dir'] 
                      [-b 'dir'] -d branding
-c channel the package channel (trunk, asan, unstable, beta, stable)
-a arch    package architecture (ia32 or x64)
-o dir     package output directory [/home/user/chromium/src/chrome/installer/linux/debian]
-b dir     build input directory    [/home/user/chromium/src/out/linux-release/]
-d brand   either chromium or google_chrome
-h         this help message
Cleaning...

Ok, now to build.

$ ./build.sh -c stable -d chromium
./build.sh: line 226: /home/user/chromium/src/out/linux-release//installer/common/installer.include: No such file or directory
Cleaning...

And indeed, the directory is missing.

$ ls chromium/src/out/linux-release/
args.gn                         gen                             libmus_library.so              protoc
bro                             genmacro                        libmus_library.so.TOC          pyproto
build.ninja                     genmodule                       libtracing_library.so          re2c
build.ninja.d                   genperf                         libtracing_library.so.TOC      resources
character_data_generator        genstring                       locales                        resources.pak
chrome                          genversion                      mksnapshot                     snapshot_blob.bin
chrome.1                        icudtl.dat                      Mojo Applications              toolchain.ninja
chrome_100_percent.pak          keyboard_resources.pak          mojo_runner                    views_mus_resources.pak
chrome_200_percent.pak          libEGL.so                       mus_app_resources_100.pak      xdg-mime
chrome-wrapper                  libEGL.so.TOC                   mus_app_resources_200.pak      xdg-settings
content_browser_manifest.json   libfont_service_library.so      mus_app_resources_strings.pak  yasm
content_gpu_manifest.json       libfont_service_library.so.TOC  natives_blob.bin
content_renderer_manifest.json  libGLESv2.so                    obj
content_utility_manifest.json   libGLESv2.so.TOC                product_logo_48.png

Maybe it's somewhere else.

$ find chromium/ -name installer -type d
chromium/src/chrome/installer
chromium/src/chrome/test/data/installer
chromium/src/remoting/host/installer
chromium/src/out/linux-release/obj/chrome/installer

Appears unrelated. 

$ ls chromium/src/out/linux-release/obj/chrome/installer
installer.ninja  util

It has the file, but doesn't copy it.

$ find chromium/ -name installer.include -type f
chromium/src/chrome/installer/linux/common/installer.include