This should avoid the 28 lines of unnecessary spam about fetching stdenv
and bash:

````
=== Building pull request NixOS#28261
these paths will be fetched (0.51 MiB download, 2.50 MiB unpacked):
  /nix/store/2pm24nzym7z11nddnf6vmqhsqmzx9h3g-bash-4.4-p12-dev
  /nix/store/3n6g5nmyn63gsgjc7z03rsd88s2q7b7i-stdenv
  /nix/store/7mpcc6x6b3db595rsqr1qjckpaj1dym8-bash-4.4-p12-info
  /nix/store/d9hb8f6j2syq08hw97qajdcdkl9lwg9n-bash-4.4-p12-doc
fetching path ‘/nix/store/d9hb8f6j2syq08hw97qajdcdkl9lwg9n-bash-4.4-p12-doc’...
*** Downloading ‘https://cache.nixos.org/nar/013kcshsm2hzpngmrf4izq3ag8s9avrkhz5z8l8mgnxay1lxlz99.nar.xz’ to ‘/nix/store/d9hb8f6j2syq08hw97qajdcdkl9lwg9n-bash-4.4-p12-doc’...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  357k  100  357k    0     0  3342k      0 --:--:-- --:--:-- --:--:-- 3408k
fetching path ‘/nix/store/2pm24nzym7z11nddnf6vmqhsqmzx9h3g-bash-4.4-p12-dev’...
*** Downloading ‘https://cache.nixos.org/nar/1225myjpymp5bv0x5zl08dmdz786jf86fshc2dwa3zgxasrppcfi.nar.xz’ to ‘/nix/store/2pm24nzym7z11nddnf6vmqhsqmzx9h3g-bash-4.4-p12-dev’...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 54312  100 54312    0     0   231k      0 --:--:-- --:--:-- --:--:--  232k
fetching path ‘/nix/store/7mpcc6x6b3db595rsqr1qjckpaj1dym8-bash-4.4-p12-info’...
*** Downloading ‘https://cache.nixos.org/nar/04dzl4jlhxhisiby9lyw2k6q2w5ns3xy1h6gfdc14qn649lshy2f.nar.xz’ to ‘/nix/store/7mpcc6x6b3db595rsqr1qjckpaj1dym8-bash-4.4-p12-info’...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  107k  100  107k    0     0  1179k      0 --:--:-- --:--:-- --:--:-- 1198k
fetching path ‘/nix/store/3n6g5nmyn63gsgjc7z03rsd88s2q7b7i-stdenv’...
*** Downloading ‘https://cache.nixos.org/nar/013xg2cnv9wf3fkhcl3b3b6iihqnwhy65m6ijb6ynwavy584w2xi.nar.xz’ to ‘/nix/store/3n6g5nmyn63gsgjc7z03rsd88s2q7b7i-stdenv’...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  8640  100  8640    0     0   143k      0 --:--:-- --:--:-- --:--:--  148k
=== Reviewing PR 28261 : busybox: 1.26.2 -> 1.27.1, fix bootstrap
````

(From https://travis-ci.org/NixOS/nixpkgs/jobs/264390932)

Travis doesn't have explicit support for labeling builds
in a build matrix, so use a dummy environment variable as a poor man's
label.

Before: https://travis-ci.org/NixOS/nixpkgs/builds/264390928
After: TBD