formula_installer: re-enable mirror usage when installing via API

[EricFromCanada]

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Since #18562, installing from source via the API doesn't download from a mirror if the original is unreachable. Re-initializing downloadable_object seems to fix this. (🎩 @Bo98)

Note that this only works for brew install -s, and not for brew fetch -s.

Before:

$ tail -n 1 /etc/hosts
127.0.0.1   ftp.gnu.org
$ brew install -s libunistring
libunistring 1.2 is already installed but outdated (so it will be upgraded).
==> Fetching libunistring
==> Downloading https://raw.githubusercontent.com/Homebrew/homebrew-core/bcf0b597f2caa3342586ca5e0d2379b2337b7a56/Formul
Already downloaded: /Users/eric/Library/Caches/Homebrew/downloads/027cce73171392f3c59ccae026aaad023c190d1dadcce6b9ee572f6a5eb2804e--libunistring.rb
==> Downloading https://ftp.gnu.org/gnu/libunistring/libunistring-1.3.tar.gz
curl: (7) Failed to connect to ftp.gnu.org port 443 after 1 ms: Couldn't connect to server

Error: libunistring: Failed to download resource "libunistring"
Download failed: https://ftp.gnu.org/gnu/libunistring/libunistring-1.3.tar.gz

After:

$ brew install -s libunistring
libunistring 1.2 is already installed but outdated (so it will be upgraded).
==> Fetching libunistring
==> Downloading https://raw.githubusercontent.com/Homebrew/homebrew-core/bcf0b597f2caa3342586ca5e0d2379b2337b7a56/Formul
Already downloaded: /Users/eric/Library/Caches/Homebrew/downloads/027cce73171392f3c59ccae026aaad023c190d1dadcce6b9ee572f6a5eb2804e--libunistring.rb
==> Downloading https://ftp.gnu.org/gnu/libunistring/libunistring-1.3.tar.gz
curl: (7) Failed to connect to ftp.gnu.org port 443 after 1 ms: Couldn't connect to server

Trying a mirror...
==> Downloading https://ftpmirror.gnu.org/libunistring/libunistring-1.3.tar.gz
==> Downloading from https://mirror.its.dal.ca/gnu/libunistring/libunistring-1.3.tar.gz
###########                                                                                                         9.8%

[Bo98]

Thanks!