Ignore temporary files in BindingStepTest.grep

[jglick]

Otherwise can get PCT flakes such as

java.nio.file.NoSuchFileException: …/target/tmp/j h8642771068129316003/jobs/p/builds/1/atomic7832996773747571639.tmp
	at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
	at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
	at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:116)
	at java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:219)
	at java.base/java.nio.file.Files.newByteChannel(Files.java:371)
	at java.base/java.nio.file.Files.newByteChannel(Files.java:422)
	at java.base/java.nio.file.spi.FileSystemProvider.newInputStream(FileSystemProvider.java:420)
	at java.base/java.nio.file.Files.newInputStream(Files.java:156)
	at org.apache.commons.io.FileUtils.lambda$readFileToString$12(FileUtils.java:2615)
	at org.apache.commons.io.IOUtils.toString(IOUtils.java:3239)
	at org.apache.commons.io.IOUtils.toString(IOUtils.java:3214)
	at org.apache.commons.io.FileUtils.readFileToString(FileUtils.java:2615)
	at org.jenkinsci.plugins.credentialsbinding.impl.BindingStepTest.grep(BindingStepTest.java:471)
	at org.jenkinsci.plugins.credentialsbinding.impl.BindingStepTest.grep(BindingStepTest.java:459)
	at org.jenkinsci.plugins.credentialsbinding.impl.BindingStepTest.lambda$cleanupAfterRestart$8(BindingStepTest.java:265)

when grepping the build directory before the build completes.