flake8_atlas fix, master branch (2019.04.17.)

Last night's inclusion of !22635 (merged) unfortunately broke the build of @fwinkl's (relatively) new flake8_atlas code's installation.

The immediate problem was that with a recent AtlasCMake update atlas_build_run.sh is now in a different place. But while fixing that, I also had to realise that in the current setup flake8_atlas was not installed alongside the release. It was only ever put into the build directory. So I fixed that as well.

To demonstrate:

[bash][lxplus712]:~ > asetup Athena,master,r15
Using Athena/22.0.2 [cmake] with platform x86_64-centos7-gcc8-opt
	at /cvmfs/atlas-nightlies.cern.ch/repo/sw/master/2019-04-15T2141
[bash][lxplus712]:~ > flake8 --version
3.6.0 (mccabe: 0.6.1, pycodestyle: 2.4.0, pyflakes: 2.0.0) CPython 2.7.15 on Linux
[bash][lxplus712]:~ >

(r15 was before the build of the package broke.)

At the same time I also started using a stamp file in the installation of the code, as we discussed in !22564 (merged) already.