Eliminate macOS sprintf warning

AppleClang 14.0.0.14000029 warns about a potential security problem
while invoking the sprintf C function:

    internal/ceres/fixed_array_test.cc:469:3: warning: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
      sprintf(buf.data(), "foo");  // NOLINT(runtime/printf)
      ^
    /Applications/Xcode_14.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk/usr/include/stdio.h:188:1: note: 'sprintf' has been explicitly marked deprecated here
    __deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
    ^
    /Applications/Xcode_14.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk/usr/include/sys/cdefs.h:215:48: note: expanded from macro '__deprecated_msg'
            #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))

Replace sprintf by snprintf to avoid this deprecation warning.

Change-Id: I6870c0bd4e390388d1d7bcec082cee272b234eba