Preserve values of environment variables which are already set

This enables the environment variables controlling run.sh behavior to be
permanently set in a working environment (e.g. to automatically force
colored output without using "-c" in each runall.sh invocation).

Relevant runall.sh command line arguments still have a higher priority.
2 jobs for tidy-runall.sh-environment-variables in 42 minutes and 37 seconds (queued for 18 minutes and 44 seconds)
Status Job ID Name Coverage
  Build
passed #1516
linux docker
build:debian:sid:amd64

00:41:40

passed #1517
linux docker
build:debian:sid:i386

00:42:34