[stable31] chore: Add nix flake with development shell #17857

Job	Run time
changes	7s
php8.1-webdav	1m 24s
php8.4-webdav	1m 7s
php8.3-webdav	1m 43s
php8.2-webdav	1m 45s
files-external-webdav-summary	7s
	6m 13s

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