Tools/src/checkstyle.sh refers to non-existent Tools/bin/goimports

This causes these error messages to be output during the build:

```
/Users/mark.sheppard/code/amazon-ssm-agent/Tools/src/checkstyle.sh: line 37: Tools/bin/goimports: No such file or directory
/Users/mark.sheppard/code/amazon-ssm-agent/Tools/src/checkstyle.sh: line 47: Tools/bin/goimports: No such file or directory
/Users/mark.sheppard/code/amazon-ssm-agent/Tools/src/checkstyle.sh: line 57: Tools/bin/goimports: No such file or directory
```

Fixing that by changing `Tools/bin/goimports` to be just `goimports` then causes the build to fail:

```
Error: Found files not formatted by goimports
/Users/mark.sheppard/code/amazon-ssm-agent/agent/fileutil/artifact/artifact_integ_test.go
/Users/mark.sheppard/code/amazon-ssm-agent/agent/plugins/dockercontainer/dockercontainer_test.go
/Users/mark.sheppard/code/amazon-ssm-agent/agent/plugins/domainjoin/domainjoin_common.go
/Users/mark.sheppard/code/amazon-ssm-agent/agent/ssm/authtokenrequest/authtokenrequest_client.go
/Users/mark.sheppard/code/amazon-ssm-agent/agent/ssm/util/util_test.go
/Users/mark.sheppard/code/amazon-ssm-agent/agent/update/processor/processor.go
Please run 'goimports -w' for files listed.
make: *** [makefile:21: checkstyle] Error 1
```

Read Tools/src/checkstyle.sh at lines 37, 47, and 57, then inspect the listed Go files reported by the checkstyle run. Run the checkstyle build target to reproduce the missing goimports errors and formatting failures; done means the command resolves correctly and the checkstyle target passes without those messages.

Written by the indexing model from the issue text.