Populate the env with dry run status

Updates the `applyEnvironmentVariables` function to populate whether or
not the current execution context is running as a dry run or not which
can then be used in hooks.

Author: jacobbednarz

doc/hooks.md

@@ -69,6 +69,7 @@ The following variables are available on all hooks:
 - `GH_OST_INSPECTED_HOST`
 - `GH_OST_EXECUTING_HOST`
 - `GH_OST_HOOKS_HINT` - copy of `--hooks-hint` value
+- `GH_OST_DRY_RUN` - whether or not the `gh-ost` run is a dry run
 
 The following variable are available on particular hooks:
 

go/logic/hooks.go

@@ -64,6 +64,7 @@ func (this *HooksExecutor) applyEnvironmentVariables(extraVariables ...string) [
 	env = append(env, fmt.Sprintf("GH_OST_INSPECTED_HOST=%s", this.migrationContext.GetInspectorHostname()))
 	env = append(env, fmt.Sprintf("GH_OST_EXECUTING_HOST=%s", this.migrationContext.Hostname))
 	env = append(env, fmt.Sprintf("GH_OST_HOOKS_HINT=%s", this.migrationContext.HooksHintMessage))
+	env = append(env, fmt.Sprintf("GH_OST_DRY_RUN=%t", this.migrationContext.Noop))
 
 	for _, variable := range extraVariables {
 		env = append(env, variable)