When concurrent misc_cache provider index rebuilds happen, try to
rebuild it only once, so we don't exceed misc_cache lock timeout.
For example, when using `spack env depfile`, with no previous
misc_cache, running `make -f depfile -j8` could run at most 8 concurrent
`spack install` locking on misc_cache to rebuild the provider index. If
one rebuild takes 30s, before this fix, the "worst" lock could wait up
to 30s * 7, easily exceeding misc_cache lock timeout. Now, the "worst"
lock would take 30s * 1 + ~1s * 6.
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