Fix print format args in RBE repo checked in configs validation

[sunjayBhatia]

Supercedes #837 as it seems to have gone stale

Taken suggestion from: #837 (comment)

We run into this as an issue when generating toolchains in a workspace with Linux and Windows toolchains, generating the Linux toolchains while there are others with checked in configs seems to hit this code and fail with: not all arguments converted during string formatting.

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[sunjayBhatia]

@googlebot I signed it!

[sunjayBhatia]

@googlebot I signed it!

[sunjayBhatia]

@googlebot I signed it!

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[sunjayBhatia]

cc @wrowe

[sunjayBhatia]

cc @smukherj1 when you get a chance

[smukherj1]

/gcbrun

[sunjayBhatia]

@smukherj1 when you get a chance looks like this is all green

[smukherj1]

huh, wonder where the prow bot is gone. It should have added the "lgtm" and "approval" labels and our auto merger should have automatically picked it when once the CI was green