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:
  golang/go#57328

Makefile

@@ -5,7 +5,7 @@ export GOFLAGS=-mod=vendor
 build: cmd/gptcli
 
 cmd/gptcli: vendor FORCE
-	go build -o gptcli cmd/gptcli/*.go
+	CGO_ENABLED=0 go build -o gptcli cmd/gptcli/*.go
 
 vendor: go.mod
 	go mod download