ARROW-12005: [R] Fix a bash typo in configure
ARROW-12005: [R] Fix a bash typo in configure

Without the quotes, we get ` parse error: condition expected: !=`

Closes #9741 from jonkeane/ARROW-12005-configure-bash-typo

Authored-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>
Signed-off-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>

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