Fix the missing "context" pacakge

[adlerhsieh]

WHAT

fix build error by fixing the missing "context" package.

WHY

• https://github.com/cloudspannerecosystem/wrench/pull/43 introduced a build error (the file is using context package but the package is not imported).
• Out projects are using the tool, and it has the error when we don't specify a version.

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[adlerhsieh]

@googlebot I signed it!