Fix the unbound variable error on all-in-one image build

[Ashmita152]

Signed-off-by: Ashmita Bohara [email protected]

Which problem is this PR solving?

When someone creates a PR from fork's master branch, first part of the if condition succeeds and then it looks for DOCKERHUB_LOGIN variable which is only set when we push to master (ie. not for pr) here: https://github.com/jaegertracing/jaeger/blob/master/.github/workflows/ci-all-in-one-build.yml#L38-L41
This is causing unbound variable error. Setting it default to false will help in solving the error.

This is currently happening for this PR: #2657

[codecov]

Codecov Report

Merging #2675 (1a2642d) into master (761facc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2675   +/-   ##
=======================================
  Coverage   95.55%   95.55%           
=======================================
  Files         214      214           
  Lines        9535     9535           
=======================================
  Hits         9111     9111           
+ Misses        346      345    -1     
- Partials       78       79    +1     

Impacted Files	Coverage Δ
cmd/query/app/server.go	88.52% <0.00%> (-1.64%)	⬇️
...lugin/sampling/strategystore/adaptive/processor.go	100.00% <0.00%> (+0.92%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 761facc...1a2642d. Read the comment docs.