build: Fix path to openapi-gen binary

[terrytangyuan]

Currently the openapi-gen binary is installed to $GOPATH/bin but we are calling it with other path. This will introduce errors when older versions of openapi-gen has installed (for use in other projects) and confuses the path.

unknown flag: --report-filename
Usage of openapi-gen:
      --alsologtostderr                  log to standard error as well as files
      --build-tag string                 A Go build tag to use to identify files generated by this command. Should be unique. (default "ignore_autogenerated")
  -h, --go-header-file string            File containing boilerplate header text. The string YEAR will be replaced with the current 4-digit year. (default "/Users/yuan.tang/go-workspace/src/k8s.io/code-generator/hack/boilerplate.go.txt")
  -i, --input-dirs strings               Comma-separated list of import paths to get input types from.
      --log_backtrace_at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
      --log_dir string                   If non-empty, write log files in this directory
      --logtostderr                      log to standard error instead of files (default true)
  -o, --output-base string               Output base; defaults to $GOPATH/src/ or ./ if $GOPATH is not set. (default "/Users/yuan.tang/go-workspace/src")
  -O, --output-file-base string          Base name (without .go suffix) for output files. (default "openapi_generated")
  -p, --output-package string            Base package path.
      --stderrthreshold severity         logs at or above this threshold go to stderr (default 2)
  -v, --v Level                          log level for V logs
      --verify-only                      If true, only verify existing output, do not write anything.
      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging
unknown flag: --report-filename
make: *** [Makefile:505: pkg/apis/workflow/v1alpha1/openapi_generated.go] Error 2

Signed-off-by: terrytangyuan terrytangyuan@gmail.com

Checklist:

• My organization is added to USERS.md.

[alexec]

$(GOPATH)?

[terrytangyuan]

I used curly brances to be consistent with other similar places in Makefile, e.g. ${GOPATH}/bin/go-to-protobuf. Would you like me to change all of them to () in this PR?

[terrytangyuan]

Just replaced all the {} by () for consistency.