releasetools: Set the search path based on the current executable.

We used to mimic the behavior of build system, to find the default
search path based on OUT_DIR_COMMON_BASE or OUT_DIR. These variables
should be internal to build system.

Since we've switched releasetools script to hermetic Python executables
(e.g. `m -j ota_from_target_files`, then run the binary at
`out/host/linux-x86/bin/ota_from_target_files`), we can set the search
path in relative to the path of the current executable.

Bug: 133126366
Test: TreeHugger
Test: 1. Build aosp_x86, by "lunch aosp_x86; m -j"
      2. Inject errors to the executables under out/host/linux-x86/bin,
         e.g.  to `lpmake`.
      3. Set up OUT_DIR (e.g., to /tmp/out) and build the same product
         again by "export OUT_DIR=/tmp/out; lunch aosp_x86; m -j". Check
         that the second run finishes successfully (with the binaries at
         /tmp/out as opposed to out/; otherwise it would fail the build
         due to the invalid binaries from step 2).
Test: lunch a target;
      `atest --host releasetools_test releasetools_py3_test`
Change-Id: I366099c3dfd5fa4282745ef258a8cf35338e1e42