golang: Allow users to specify desired buildflags

For example one could want to build the software with race detector,
e.g.

	[gowork]
	...
	buildflags = -race

or if you want to build a plugin for fluent-bit

	[gowork]
	...
	install	   = your.package.import.path
	buildflags = -buildmode=c-shared

and then your package will be installed as shared library into

	${gowork:directory}/pkg/`go env GOOS`_`go env GOARCH`/<your.package.import.path>.a

One could pass that location explicitly to `fluentd-bit -e ...` or
symlink to it from some .so from global directory of fluent-bit plugins.

Maybe it is not very good to use `go install` for building plugins
(instead of `go build -o ...`) but we anyway need to have ability to
specify buildflags as a generic feature, hence this patch.

/reviewed-on nexedi/slapos!274

component/golang/buildout.cfg

@@ -63,6 +63,11 @@ environment-extra =
 #       lab.nexedi.com/kirr/neo/go/...  \
 #       github.com/pkg/profile          \
 #       golang.org/x/perf/cmd/benchstat
+#
+# it is possible to specify Go build flags used for compilation e.g. this way:
+#
+#   [gowork]
+#   buildflags = -race
 [gowork]
 directory = ${buildout:directory}/go.work
 src	= ${:directory}/src
@@ -72,6 +77,9 @@ depends = ${gowork.goinstall:recipe}
 # go version used for the workspace (possible to override in applications)
 golang  = ${golang19:location}
 
+# no special build flags by default
+buildflags =
+
 # everything is done by dependent parts
 recipe  = plone.recipe.command
 command = :
@@ -100,7 +108,7 @@ stop-on-error = true
 # clients should put package list to install to gowork:install ("..." requests installing everything)
 [gowork.goinstall]
 recipe  = plone.recipe.command
-command = bash -c ". ${gowork:env.sh} && go install -v ${gowork:install}"
+command = bash -c ". ${gowork:env.sh} && go install ${gowork:buildflags} -v ${gowork:install}"
 update-command = ${:command}
 stop-on-error = true