selinux_linux: fix path in execLabel()

[mschiff]

execLabel() was trying to read /proc/thread-self/exec instead of /proc/thread-self/attr/exec

[kolyshkin]

@mschiff thank you! The fix is correct (and the issue is introduced in commit c8cfa6f). Unfortunately it was missed during a review, and we don't have a test case to catch that.

I have force-pushed to your branch to fix the missing Signed-off-by and add a second commit with the test case.

The test fails before the fix like this:

=== RUN   TestExecLabel
    selinux_linux_test.go:236: open /proc/thread-self/exec handle: unsafe procfs detected: openat2 /proc/thread-self/exec: no such file or directory
--- FAIL: TestExecLabel (0.00s)

[kolyshkin]

@thaJeztah PTAL

[thaJeztah]

LGTM, thx!