Makefile: don't hardcode the path for bash (#1086)

Some systems (e.g., NixOS) don't have `/bin/bash`,
so use the `env` utility to locate `bash` instead. 813015b

doc/ixgbe/virtual_function/fn.create_virtual_nic.html

@@ -3,7 +3,7 @@
     ip_addresses: <a class="struct" href="https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html" title="struct alloc::vec::Vec">Vec</a>&lt;[<a class="primitive" href="https://doc.rust-lang.org/nightly/core/primitive.u8.html">u8</a>; <a class="primitive" href="https://doc.rust-lang.org/nightly/core/primitive.array.html">4</a>]&gt;,
     default_rx_queue: <a class="primitive" href="https://doc.rust-lang.org/nightly/core/primitive.usize.html">usize</a>,
     default_tx_queue: <a class="primitive" href="https://doc.rust-lang.org/nightly/core/primitive.usize.html">usize</a>
-) -&gt; <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;VirtualNic&lt;IxgbeRxQueueRegisters, AdvancedRxDescriptor, IxgbeTxQueueRegisters, AdvancedTxDescriptor&gt;, &amp;'static <a class="primitive" href="https://doc.rust-lang.org/nightly/core/primitive.str.html">str</a>&gt;</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Create a virtual NIC from the ixgbe device.</p>
+) -&gt; <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a>&lt;<a class="struct" href="../../virtual_nic/struct.VirtualNic.html" title="struct virtual_nic::VirtualNic">VirtualNic</a>&lt;IxgbeRxQueueRegisters, AdvancedRxDescriptor, IxgbeTxQueueRegisters, AdvancedTxDescriptor&gt;, &amp;'static <a class="primitive" href="https://doc.rust-lang.org/nightly/core/primitive.str.html">str</a>&gt;</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Create a virtual NIC from the ixgbe device.</p>
 <h2 id="arguments"><a href="#arguments">Arguments</a></h2>
 <ul>
 <li><code>nic_id</code>: the ixgbe NIC we will take receive and transmit queue from.</li>