In BF-30343:

• Originally, the ${registry_username} was getting expanded to the value defined in the previous function since evergreen expansion is executed before bash interpretation
• After the evergreen update in EVG-20276, the expansion the previous function defines registry_username is getting reset to an empty string
• Even though bash would've properly interpreted ${registry_username}, since evergreen is expanding the variable before bash gets a chance to see, it's left with an empty string

Evergreen only expands variables wrapped in curly braces, so switching from ${registry_username} to $registry_username prevents evergreen from expanding registry_username and gives bash the chance to actually interpret it as a bash variable.