Update source install instructions

[chapulina]

🦟 Bug fix

Summary

I was following the source install instructions for just Gazebo from https://ignitionrobotics.org/api/gazebo/6.4/install.html and noticed it needed some fixes:

• Install necessary tools
• Clone repository only once
• Install Ignition dependencies from debs (since these instructions are just for building Gazebo)

We should eventually port this tutorial to Citadel / Edifice. It was originally added in #1078.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #1311 (1810532) into ign-gazebo6 (7a931c9) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo6    #1311      +/-   ##
===============================================
- Coverage        62.25%   62.25%   -0.01%     
===============================================
  Files              278      278              
  Lines            23203    23203              
===============================================
- Hits             14446    14445       -1     
- Misses            8757     8758       +1     

Impacted Files	Coverage Δ
src/EntityComponentManager.cc	88.26% <0.00%> (-0.13%)	⬇️

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-01-citadel-edifice-fortress/1313/1