fix: recognize kernel mount failure when target already mounted.

[smoser]

In a single stacker file that used a squashfs image twice or more, stacker would mount the layers the first time correctly. The image build would fail. The reason was that maybeKernelSquashMount did not detected why the mount failed (it was already mounted), but only that it did fail. So the code then attempted to extract the squashfs layer with unsquashfs and an error would occur like:

FATAL ERROR: dir_scan: failed to change permissions for directory
.../roots/sha256_52068a5d6c1...02caeb3a706cba3a/overlay,
because Read-only file system
Parallel unsquashfs: Using 32 processors
691 inodes (966 blocks) to write

The error only actually occurs if:

• user is priveleged (can do a kernel mount)
• stacker is building squash images (--layer-type=squashfs)
• user does not have squashfuse in their path.

This is fixed more correctly upstream in 565b032.

An example stacker file that would recreate:

b1:
  build_only: true
  from:
    type: docker
    url: oci:my-ocidir:base:latest-squashfs
run: |
  echo "hello world" > f1

b2:
  from:
    type: docker
    url: oci:my-ocidir:base:latest-squashfs
  run: |
    echo "goodbye" > f2

The fix here is not great... effectively grepping the output of 'mount'
for 'is already mounted'. We set the LANG environment variable to C
to avoid failures due to a translations.

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