ci(gcb): print info about machine doing local build (#6123)
ci(gcb): print info about machine doing local build (#6123)

These builds can run in a variety of environments, in Google Cloud
Build, a local docker, or on the local machine. It can be useful to
see some stats about the local machine runing the build.

Example output from a cloud build run:

```
Step #2: ====================
Step #2: |   Machine Info   |
Step #2: ====================
Step #2:     local: Sat Mar 27 17:32:03 UTC 2021 (+0000)
Step #2:       utc: Sat Mar 27 17:32:03 UTC 2021
Step #2:    kernel: #41~18.04.1-Ubuntu SMP Fri Feb 26 22:23:13 UTC 2021
Step #2:        os: PRETTY_NAME="Fedora 33 (Container Image)"
Step #2:     nproc: 32
Step #2:       mem: 28.21 GiB
Step #2:       gcc: gcc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)
Step #2:     clang: clang version 11.0.0 (Fedora 11.0.0-2.fc33)
Step #2:        cc: cc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)
Step #2:     cmake: cmake version 3.19.7
Step #2:     bazel: bazel 3.5.0
```
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp