Avoid chdir in subshells

The preferred "chdir for this step only" idiom is to use the block-form
'cd' method.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/macvim.rb

@@ -58,7 +58,9 @@ class Macvim < Formula
     end
 
     # Reference: https://github.com/b4winckler/macvim/wiki/building
-    system "cd src/MacVim/icons && make getenvy"
+    cd 'src/MacVim/icons' do
+      system "make getenvy"
+    end
 
     system "make"
 

Formula/qi.rb

@@ -17,11 +17,17 @@ class Qi < Formula
 
   def install
     if ARGV.include? '--SBCL'
-      system "cd Lisp; sbcl --load 'install.lsp'"
+      cd 'Lisp' do
+        system "sbcl", "--load", "install.lsp"
+      end
+
       system "echo \"#!/bin/bash\nsbcl --core #{prefix}/Qi.core $*\" > qi"
       prefix.install ['Lisp/Qi.core']
     else
-      system "cd Lisp; clisp install.lsp"
+      cd 'Lisp' do
+        system "clisp", "install.lsp"
+      end
+
       system "echo \"#!/bin/bash\nclisp -M #{prefix}/Qi.mem $*\" > qi"
       prefix.install ['Lisp/Qi.mem']
     end

Formula/samtools.rb

@@ -9,7 +9,9 @@ class Samtools < Formula
   def install
     system "make"
     system "make razip"
-    system "cd bcftools; make"
+    cd 'bcftools' do
+      system "make"
+    end
 
     bin.install %w{samtools razip bcftools/bcftools bcftools/vcfutils.pl}
     bin.install %w{misc/maq2sam-long misc/maq2sam-short misc/md5fa misc/md5sum-lite misc/seqtk misc/wgsim}