docs: fix misleading example of setting an env variable for a single command

[muru]

- What I did

Fix a misleading example of setting an environment variable for a single command.

- How I did it

The DEBIAN_FRONTEND environment variable is used to control the interface by which debconf questions are presented to the user (see man 7 debconf). In DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y, the DEBIAN_FRONTEND environment variable is only set for the apt-get update command which does not ask debconf questions, and will not affect the apt-get install command where these questions are actually asked. It should be the other way around.

- How to verify it

The way environment variables work for single commands can be tested thus:

% foo=bar sh -c 'echo "$0: |$foo|"' first_shell && sh -c 'echo "$0: |$foo|"' second_shell
first_shell: |bar|
second_shell: ||
% sh -c 'echo "$0: |$foo|"' first_shell && foo=bar sh -c 'echo "$0: |$foo|"' second_shell
first_shell: ||
second_shell: |bar|
% (export foo=bar; sh -c 'echo "$0: |$foo|"' first_shell && sh -c 'echo "$0: |$foo|"' second_shell)    
first_shell: |bar|
second_shell: |bar|

(In the last example, I'm running the commands in a subshell to avoid affecting the environment of my current shell.)

- Description for the changelog
Fix misleading example of setting an environment variable for a single command

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #3680 (7227c01) into master (c59773f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3680   +/-   ##
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  Coverage   59.01%   59.01%           
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  Files         289      289           
  Lines       24623    24623           
=======================================
  Hits        14532    14532           
  Misses       9218     9218           
  Partials      873      873           

[thaJeztah]

LGTM

nice catch, thanks!