📖 Add envsubst as prerequisite in developer guide

[Insullone]

Add envsubst in Chapter 'Developer Guide' of the book.
Also update corresponding instructions using envsubst.

Fixes #4012

[k8s-ci-robot]

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[fabriziopandini]

/ok-to-test
/milestone v0.3.13

[randomvariable]

Would be good to add a comment that GNU gettext is insufficient (we've noticed some parsing differences in AWS templates, for example).

[Insullone]

@randomvariable Sure, no problem.

[Insullone]

/retest

[vincepri]

/retest

[randomvariable]

/lgtm

[fabriziopandini]

/approve

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[vincepri]

/milestone v0.3.x