[CI] Change method of password input to docker login.

[NlightNFotis]

Change how the password is being passed to the
docker login command to avoid some security issues
associated with the logging of the password to the
shell history or logs.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[piotr-grabalski]

I presume this works same way as docker login --username=dbcicprover --password-stdin < echo ${{ secrets.DOCKERHUB_ACCESS_DB_CI_CPROVER }} so LGTM

[codecov]

Codecov Report

Merging #5771 (acee1d0) into develop (cb9f69a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5771   +/-   ##
========================================
  Coverage    69.67%   69.67%           
========================================
  Files         1246     1246           
  Lines       100856   100856           
========================================
  Hits         70276    70276           
  Misses       30580    30580           

Flag	Coverage Δ
cproversmt2	43.39% <ø> (ø)
regression	66.65% <ø> (ø)
unit	32.21% <ø> (ø)

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