npm run *-watch doesn't create build/ folder on first run

[fabiodrg]

Just a small inconvenience, but when starting development from scratch running e.g. npm run firefox-watch doesn't create the build/ folder automatically. I had to change a file to trigger the build process. Perhaps, we could force an initial build when any of the watch commands are executed.
I am thinking this is also useful on colaborative Git flows:

• I have no watcher running, but have a build/ folder with my current WIP
• I checkout to another branch for review purposes or whatever
• I run the watch command and sideload the content from build/ on my browser
• Problem: I am loading my work version instead of the branch currently checked out