Add script for Cirrus CI for testing on FreeBSD

[giordano]

Like JuliaAstro/FITSIO.jl#109, but at least here I checked that the package is built for FreeBSD

[codecov-io]

Codecov Report

Merging #40 into master will decrease coverage by 2.64%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   91.62%   88.98%   -2.65%     
==========================================
  Files          21       21              
  Lines         681      681              
==========================================
- Hits          624      606      -18     
- Misses         57       75      +18

Impacted Files	Coverage Δ
src/d.jl	67.3% <0%> (-23.08%)	⬇️
src/u.jl	84.61% <0%> (-7.7%)	⬇️
src/s.jl	78.43% <0%> (-3.93%)	⬇️
src/p.jl	94.23% <0%> (-1.93%)	⬇️

---

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[coveralls]

Pull Request Test Coverage Report for Build 128

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 18 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-2.6%) to 88.987%

---

Files with Coverage Reduction	New Missed Lines	%
src/u.jl	1	84.62%
src/s.jl	2	78.43%
src/p.jl	3	94.23%
src/d.jl	12	67.31%

Totals
Change from base Build 126:	-2.6%
Covered Lines:	606
Relevant Lines:	681

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 128

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 18 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-2.6%) to 88.987%

---

Files with Coverage Reduction	New Missed Lines	%
src/u.jl	1	84.62%
src/s.jl	2	78.43%
src/p.jl	3	94.23%
src/d.jl	12	67.31%

Totals
Change from base Build 126:	-2.6%
Covered Lines:	606
Relevant Lines:	681

---

💛 - Coveralls

[helgee]

LGTM