Fix for temporary files in custom VFS. #127

Job	Run time
Compile sqlite3 for windows-latest	7s
Compile sqlite3 for ubuntu-latest	3s
Compile sqlite3 for macos-latest	7s
Analyze on Dart stable	26s
Analyze on Dart stable	16s
Unit tests with Dart stable on ubuntu-latest with system sqlite	30s
Unit tests with Dart stable on ubuntu-latest with compiled sqlite	2m 1s
Unit tests with Dart dev on ubuntu-latest with system sqlite	37s
Unit tests with Dart dev on ubuntu-latest with compiled sqlite	2m 28s
Unit tests with Dart stable on windows-latest with system sqlite	44s
Unit tests with Dart stable on windows-latest with compiled sqlite	1m 22s
Unit tests with Dart dev on windows-latest with system sqlite	1m 14s
Unit tests with Dart dev on windows-latest with compiled sqlite	57s
Unit tests with Dart stable on macos-latest with system sqlite	32s
Unit tests with Dart stable on macos-latest with compiled sqlite	36s
Unit tests with Dart dev on macos-latest with system sqlite	34s
Unit tests with Dart dev on macos-latest with compiled sqlite	41s
integration_test_macos	0s
integration_test_linux	0s
integration_test_ios	0s
	13m 15s

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