build: Ignore existing installation when running installPipPackageTask

[jackgerrits]

When iterating in a dev environment this ensures that it always overwrites the already installed version.

-I, --ignore-installed      Ignore the installed packages, overwriting them. This can break your system if the existing package is
                              of a different version or was installed with a different package manager!

[welcome]

💖 Thanks for opening your first pull request! 💖 We use semantic commit messages to streamline the release process. Before your pull request can be merged, you should make sure your first commit and PR title start with a semantic prefix. This helps us to create release messages and credit you for your hard work!
Examples of commit messages with semantic prefixes:

• fix: Fix LightGBM crashes with empty partitions
• feat: Make HTTP on Spark back-offs configurable
• docs: Update Spark Serving usage
• build: Add codecov support
• perf: improve LightGBM memory usage
• refactor: make python code generation rely on classes
• style: Remove nulls from CNTKModel
• test: Add test coverage for CNTKModel

Make sure to check out the developer guide for guidance on testing your change.

[jackgerrits]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #895 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #895      +/-   ##
==========================================
+ Coverage   85.31%   85.36%   +0.05%     
==========================================
  Files         189      189              
  Lines        8769     8769              
  Branches      558      558              
==========================================
+ Hits         7481     7486       +5     
+ Misses       1288     1283       -5     

Impacted Files	Coverage Δ
...com/microsoft/ml/spark/cognitive/RESTHelpers.scala	50.00% <0.00%> (-15.39%)	⬇️
...a/com/microsoft/ml/spark/io/http/HTTPClients.scala	73.21% <0.00%> (+16.07%)	⬆️

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[welcome]

Congrats on merging your first pull request, we appreciate your support! 🎉🎉🎉