fix(kube-runtime): setup backoff with builder pattern

[tiagolobocastro]

Filling the fields manually and leaving the current_interval out meant that the current_interval is set to its default rather than track the initial_interval
The builder addresses this for us by setting it on the build method.

Motivation

Noticed that the current_interval of the watcher backoff is left empty, which may lead to unintended behaviour.
Granted the difference is currently minimal (800ms to 500ms)

Solution

Using the builder pattern method from the backoff crate can be used to set the fields appropriately rather than simply the default value.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.2%. Comparing base (a6060c4) to head (a181dec).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kube-runtime/src/watcher.rs	0.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1603     +/-   ##
=======================================
+ Coverage   75.2%   75.2%   +0.1%     
=======================================
  Files         82      82             
  Lines       7336    7335      -1     
=======================================
  Hits        5514    5514             
+ Misses      1822    1821      -1     

Files with missing lines	Coverage Δ
kube-runtime/src/watcher.rs	44.4% <0.0%> (+0.2%)	⬆️

[clux]

Thank you!

[tiagolobocastro]

Thanks @clux, I've fixed the fmt and also added a test, so hopefully code cov is also ok now.