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:

```
$ spotsh
spotsh: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not
  found (required by spotsh)
spotsh: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not
  found (required by spotsh)
```

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/spotsh
 
 cmd/spotsh: FORCE
-	go build -o spotsh cmd/spotsh/*.go
+	CGO_ENABLED=0 go build -o spotsh cmd/spotsh/*.go
 
 TESTPKGS=github.com/mikeb26/spotsh/internal github.com/mikeb26/spotsh/internal/aws