gptcli (v0.1.2)

Build w/ CGO_ENABLED=0

The CircleCI go build images' toolchain depends on a version of GLIBC
that is higher than the version that ships on Ubuntu 20.04. This means
users trying to install the released binaries on Ubuntu 20.04 will
see:

```
$ gptcli
gptcli: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not
  found (required by gptcli)

gptcli: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not
  found (required by gptcli)
```

This commit attempts to mitigate the problem by building w/
CGO_ENABLED=0 as suggested by:
  https://github.com/golang/go/issues/57328

gptcli (v0.1.1)

README: Update install instructions