cleanup(ci): use tf variable where possible

[dbolduc]

Someone is copying the terraform configuration, but with a different project. They noticed the project name could be substituted here.

google-cloud-rust/.gcb/builds/variables.tf

Lines 15 to 17 in b412e6c

	variable "project" {
	default = "rust-sdk-testing"
	}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.57%. Comparing base (b412e6c) to head (4b2cb3b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #805   +/-   ##
=======================================
  Coverage   95.57%   95.57%           
=======================================
  Files          36       36           
  Lines        1357     1357           
=======================================
  Hits         1297     1297           
  Misses         60       60           

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