travis: Fix the cargotest bot

Recently the Cargo revision running on cargotest was updated in #38051, and the
updated version of Cargo pulls in the openssl-sys 0.9 crate instead of the old
openssl-sys 0.7 crate. The 0.9 release of openssl-sys has slightly different
requirements and logic for detecting OpenSSL, namely it requires `pkg-config` to
be present on Linux typically.

This commit fixes this problem by installing pkg-config in the cargotest
container that's running on the bots. This in turn should hopefully fix the
build script and allow it to find the already-installed local OpenSSL libraries.

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