fix: correct shell substitution in installer

[mdqst]

Description:
This pull request addresses a small issue in the script where there was a typographical error in the line:

FOUNDRY_DIR="${FOUNDRY_DIR-"$BASE_DIR/.foundry"}"

The correct syntax for setting a default value using parameter expansion should be:

FOUNDRY_DIR="${FOUNDRY_DIR:-"$BASE_DIR/.foundry"}"

Importance:

• The original code incorrectly used {" instead of :-, which is the correct syntax for default value assignment in Bash parameter expansion.
• This error would prevent the script from correctly setting the FOUNDRY_DIR variable if it wasn't previously defined.
• The fix ensures that if FOUNDRY_DIR is not set, it will default to "$BASE_DIR/.foundry", which is the intended behavior.

By correcting this, the script now functions as expected, ensuring proper assignment of the FOUNDRY_DIR variable and avoiding potential issues in its execution.

[DaniPopes]

They're both correct, but :- also substitutes if the variable is empty (""). Thanks!