fix to support newer conda-build versions

[cdeepali]

Checklist before submitting

• Did you read the contributor guide?
• Did you update any affected documentation?
• Did you write any tests to validate this change?

Description

With conda-build versions higher than 3.21.7, we are seeing the issue described in #117 with default value of config.croot.
Thus setting config.croot to blank here, to avoid the error and thus support newer versions of conda-build.

I have retained pins in Dockerfiles, to ensure that the build is done against same version of conda-build across the board.

Review process to land

1. All tests and other checks must succeed.
2. At least one maintainer must review and approve.
3. If any maintainer requests changes, they must be addressed.

[codecov-commenter]

Codecov Report

Merging #142 (ab71b26) into main (d3a0954) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #142   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files          23       23           
  Lines        2109     2110    +1     
=======================================
+ Hits         2028     2029    +1     
  Misses         81       81           

Flag	Coverage Δ
unittests	96.16% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
open_ce/conda_utils.py	98.88% <100.00%> (+0.01%)	⬆️

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